Document the 'databases' homeserver config option (#13212)

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diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md

index ef411c5356..5deabb53d7 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -1257,6 +1257,98 @@ database:
     cp_max: 10
 ```
 ---
+### `databases`
+
+The `databases` option allows specifying a mapping between certain database tables and
+database host details, spreading the load of a single Synapse instance across multiple
+database backends. This is often referred to as "database sharding". This option is only
+supported for PostgreSQL database backends.
+
+**Important note:** This is a supported option, but is not currently used in production by the
+Matrix.org Foundation. Proceed with caution and always make backups.
+
+`databases` is a dictionary of arbitrarily-named database entries. Each entry is equivalent
+to the value of the `database` homeserver config option (see above), with the addition of
+a `data_stores` key. `data_stores` is an array of strings that specifies the data store(s)
+(a defined label for a set of tables) that should be stored on the associated database
+backend entry.
+
+The currently defined values for `data_stores` are:
+
+* `"state"`: Database that relates to state groups will be stored in this database.
+
+  Specifically, that means the following tables:
+  * `state_groups`
+  * `state_group_edges`
+  * `state_groups_state`
+
+  And the following sequences:
+  * `state_groups_seq_id`
+
+* `"main"`: All other database tables and sequences.
+
+All databases will end up with additional tables used for tracking database schema migrations
+and any pending background updates. Synapse will create these automatically on startup when checking for
+and/or performing database schema migrations.
+
+To migrate an existing database configuration (e.g. all tables on a single database) to a different
+configuration (e.g. the "main" data store on one database, and "state" on another), do the following:
+
+1. Take a backup of your existing database. Things can and do go wrong and database corruption is no joke!
+2. Ensure all pending database migrations have been applied and background updates have run. The simplest
+   way to do this is to use the `update_synapse_database` script supplied with your Synapse installation.
+
+   ```sh
+   update_synapse_database --database-config homeserver.yaml --run-background-updates
+   ```
+
+3. Copy over the necessary tables and sequences from one database to the other. Tables relating to database
+   migrations, schemas, schema versions and background updates should **not** be copied.
+
+   As an example, say that you'd like to split out the "state" data store from an existing database which
+   currently contains all data stores.
+
+   Simply copy the tables and sequences defined above for the "state" datastore from the existing database
+   to the secondary database. As noted above, additional tables will be created in the secondary database
+   when Synapse is started.
+
+4. Modify/create the `databases` option in your `homeserver.yaml` to match the desired database configuration.
+5. Start Synapse. Check that it starts up successfully and that things generally seem to be working.
+6. Drop the old tables that were copied in step 3.
+
+Only one of the options `database` or `databases` may be specified in your config, but not both.
+
+Example configuration:
+
+```yaml
+databases:
+  basement_box:
+    name: psycopg2
+    txn_limit: 10000
+    data_stores: ["main"]
+    args:
+      user: synapse_user
+      password: secretpassword
+      database: synapse_main
+      host: localhost
+      port: 5432
+      cp_min: 5
+      cp_max: 10
+
+  my_other_database:
+    name: psycopg2
+    txn_limit: 10000
+    data_stores: ["state"]
+    args:
+      user: synapse_user
+      password: secretpassword
+      database: synapse_state
+      host: localhost
+      port: 5432
+      cp_min: 5
+      cp_max: 10
+```
+---
 ## Logging ##
 Config options related to logging.