Fix missing field in AS documentation (#14845)

* Fix missing field in AS documentation

The [AS Configuration Snippet](https://matrix-org.github.io/synapse/latest/application_services.html) is missing `id` field, without it Synapse will fail to load:

```
synapse-synapse-main-0 synapse 2023-01-13 23:05:25,450 - synapse.storage.databases - 84 - INFO - main - [database config 'master']: Starting 'main' database
synapse-synapse-main-0 synapse 2023-01-13 23:05:25,452 - synapse.config.appservice - 79 - ERROR - main - Failed to load appservice from '/as/synapse-hookshot-as/registration.yaml'
synapse-synapse-main-0 synapse 2023-01-13 23:05:25,452 - synapse.config.appservice - 80 - ERROR - main - "Required string field: 'id' (/as/synapse-hookshot-as/registration.yaml)"
synapse-synapse-main-0 synapse Traceback (most recent call last):
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/config/appservice.py", line 57, in load_appservices
synapse-synapse-main-0 synapse     appservice = _load_appservice(hostname, yaml.safe_load(f), config_file)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/config/appservice.py", line 91, in _load_appservice
synapse-synapse-main-0 synapse     raise KeyError(
synapse-synapse-main-0 synapse KeyError: "Required string field: 'id' (/as/synapse-hookshot-as/registration.yaml)"
synapse-synapse-main-0 synapse 2023-01-13 23:05:25,452 - synapse.app._base - 207 - ERROR - main - Exception during startup
synapse-synapse-main-0 synapse Traceback (most recent call last):
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/app/homeserver.py", line 340, in setup
synapse-synapse-main-0 synapse     hs.setup()
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/server.py", line 310, in setup
synapse-synapse-main-0 synapse     self.datastores = Databases(self.DATASTORE_CLASS, self)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/__init__.py", line 93, in __init__
synapse-synapse-main-0 synapse     main = main_store_class(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/__init__.py", line 139, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/events_bg_updates.py", line 98, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/devices.py", line 1584, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/devices.py", line 89, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/roommember.py", line 1494, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/room.py", line 1827, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/room.py", line 1365, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/room.py", line 119, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/registration.py", line 2158, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/presence.py", line 67, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/presence.py", line 48, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/transactions.py", line 73, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/state.py", line 666, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/state.py", line 82, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/state.py", line 470, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/event_federation.py", line 2007, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/media_repository.py", line 148, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/media_repository.py", line 68, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/push_rule.py", line 330, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/event_push_actions.py", line 1938, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/metrics.py", line 68, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/event_push_actions.py", line 249, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/end_to_end_keys.py", line 1181, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/search.py", line 426, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/search.py", line 137, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/account_data.py", line 64, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/push_rule.py", line 114, in __init__
synapse-synapse-main-0 synapse     super().__init__(database, db_conn, hs)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/appservice.py", line 76, in __init__
synapse-synapse-main-0 synapse     self.services_cache = load_appservices(
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/config/appservice.py", line 57, in load_appservices
synapse-synapse-main-0 synapse     appservice = _load_appservice(hostname, yaml.safe_load(f), config_file)
synapse-synapse-main-0 synapse   File "/usr/local/lib/python3.9/site-packages/synapse/config/appservice.py", line 91, in _load_appservice
synapse-synapse-main-0 synapse     raise KeyError(
synapse-synapse-main-0 synapse KeyError: "Required string field: 'id' (/as/synapse-hookshot-as/registration.yaml)"
synapse-synapse-main-0 synapse ******************************************************************************
synapse-synapse-main-0 synapse  Error during initialisation:
synapse-synapse-main-0 synapse     "Required string field: 'id' (/as/synapse-hookshot-as/registration.yaml)"
synapse-synapse-main-0 synapse  There may be more information in the logs.
synapse-synapse-main-0 synapse ******************************************************************************
```

* Changelog

1 files changed, 1 insertions, 0 deletions

diff --git a/docs/application_services.md b/docs/application_services.md
index e4592010a2..1f988185a9 100644
--- a/docs/application_services.md
+++ b/docs/application_services.md
@@ -15,6 +15,7 @@ app_service_config_files:
 The format of the AS configuration file is as follows:
 
 ```yaml
+id: <your-AS-id>
 url: <base url of AS>
 as_token: <token AS will add to requests to HS>
 hs_token: <token HS will add to requests to AS>