Add documentation for the spam checker module (#6906)

Add documentation for the spam checker.

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+# Handling spam in Synapse
+
+Synapse has support to customize spam checking behavior. It can plug into a
+variety of events and affect how they are presented to users on your homeserver.
+
+The spam checking behavior is implemented as a Python class, which must be
+able to be imported by the running Synapse.
+
+## Python spam checker class
+
+The Python class is instantiated with two objects:
+
+* Any configuration (see below).
+* An instance of `synapse.spam_checker_api.SpamCheckerApi`.
+
+It then implements methods which return a boolean to alter behavior in Synapse.
+
+There's a generic method for checking every event (`check_event_for_spam`), as
+well as some specific methods:
+
+* `user_may_invite`
+* `user_may_create_room`
+* `user_may_create_room_alias`
+* `user_may_publish_room`
+
+The details of the each of these methods (as well as their inputs and outputs)
+are documented in the `synapse.events.spamcheck.SpamChecker` class.
+
+The `SpamCheckerApi` class provides a way for the custom spam checker class to
+call back into the homeserver internals. It currently implements the following
+methods:
+
+* `get_state_events_in_room`
+
+### Example
+
+```python
+class ExampleSpamChecker:
+    def __init__(self, config, api):
+        self.config = config
+        self.api = api
+
+    def check_event_for_spam(self, foo):
+        return False  # allow all events
+
+    def user_may_invite(self, inviter_userid, invitee_userid, room_id):
+        return True  # allow all invites
+
+    def user_may_create_room(self, userid):
+        return True  # allow all room creations
+
+    def user_may_create_room_alias(self, userid, room_alias):
+        return True  # allow all room aliases
+
+    def user_may_publish_room(self, userid, room_id):
+        return True  # allow publishing of all rooms
+```
+
+## Configuration
+
+Modify the `spam_checker` section of your `homeserver.yaml` in the following
+manner:
+
+`module` should point to the fully qualified Python class that implements your
+custom logic, e.g. `my_module.ExampleSpamChecker`.
+
+`config` is a dictionary that gets passed to the spam checker class.
+
+### Example
+
+This section might look like:
+
+```yaml
+spam_checker:
+  module: my_module.ExampleSpamChecker
+  config:
+    # Enable or disable a specific option in ExampleSpamChecker.
+    my_custom_option: true
+```
+
+## Examples
+
+The [Mjolnir](https://github.com/matrix-org/mjolnir) project is a full fledged
+example using the Synapse spam checking API, including a bot for dynamic
+configuration.