Specify PresenceRouter class method definitions up front

docs/presence_router_module.md

@@ -26,6 +26,55 @@ which can be given a list of local or remote MXIDs to broadcast known, online us
 presence to (for those users that the receiving user is considered interested in). 
 It does not include state for users who are currently offline.
 
+### Module structure
+
+Below is a list of possible methods that can be implemented, and whether they are
+required.
+
+#### `parse_config`
+
+```python
+def parse_config(config_dict: dict) -> Any
+```
+**Required.** A static method that is passed a dictionary of config options, and
+  should return a validated config object. This method is described further in
+  [Configuration](#configuration).
+
+#### `get_users_for_states`
+
+```python
+async def get_users_for_states(
+    self,
+    state_updates: Iterable[UserPresenceState],
+) -> Dict[str, Set[UserPresenceState]]:
+```
+
+An asynchronous class method that is passed an iterable of user presence state. This
+method can determine whether a given presence update should be sent to certain users.
+It does this by returning a dictionary with keys representing local or remote Matrix
+User IDs, and values being a python Set of `synapse.handlers.presence.
+UserPresenceState` instances.
+
+Synapse will then attempt to send the specified presence updates to each user when
+possible.
+
+#### `get_interested_users`
+
+```python
+async def get_interested_users(self, user_id: str) -> Union[Set[str], str]
+```
+
+An asynchronous class method that is passed a single Matrix User ID. This method is
+expected to return the users that the passed in user may be interested in the presence of.
+This may be local or remote users. It does so by returning a python Set of Matrix User
+IDs, or the string `"ALL"` to mean that the passed user should receive presence
+information for *all* known users.
+
+For clarity, if the user `@alice:example.org` is passed to this method, and the Set
+`{"@bob:example.com", "@charlie:somewhere.org"}` is returned, this signifies that Alice
+should receive presence updates sent by Bob and Charlie, regardless of whether these
+users share a room.
+
 ### Example
 
 Below is an example implementation of a presence router class.