Re-add docstring, with caveats detailed

synapse/handlers/room_member.py

@@ -514,7 +514,7 @@ class RoomMemberHandler(object):
         if old_room and old_room["is_public"]:
             yield self.store.set_room_is_public(old_room_id, False)
             yield self.store.set_room_is_public(room_id, True)
-            
+
         # Check if any groups we own contain the predecessor room
         local_group_ids = yield self.store.get_local_groups_for_room(old_room_id)
         for group_id in local_group_ids:

synapse/storage/data_stores/main/group_server.py

@@ -552,7 +552,7 @@ class GroupServerStore(SQLBaseStore):
             keyvalues={"group_id": group_id, "role_id": role_id, "user_id": user_id},
             desc="remove_user_from_summary",
         )
-    
+
     def get_local_groups_for_room(self, room_id):
         """Get all of the local group that contain a given room
         Args:

synapse/storage/data_stores/main/state.py

@@ -285,7 +285,11 @@ class StateGroupWorkerStore(
             room_id (str)
 
         Returns:
-            Deferred[unicode|None]: predecessor room id
+            Deferred[dict|None]: A dictionary containing the structure of the predecessor
+                field from the room's create event. The structure is subject to other servers,
+                but it is expected to be:
+                    * room_id (str): The room ID of the predecessor room
+                    * event_id (str): The ID of the tombstone event in the predecessor room
 
         Raises:
             NotFoundError if the room is unknown