Apply suggestions from code review

Co-authored-by: Eric Eastwood <erice@element.io>

synapse/handlers/room_member.py

@@ -309,6 +309,8 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         # the table `current_state_events` and `get_current_state_events` is `None`.
         await self.store.forget(user_id, room_id)
 
+        # If everyone locally has left the room, then there is no reason for us to keep the
+        # room around and we automatically purge room after a little bit
         if self._purge_retention_period and await self.store.is_locally_forgotten_room(
             room_id
         ):

synapse/storage/databases/main/roommember.py

@@ -1343,7 +1343,7 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
         )
 
     async def get_rooms_to_purge(self) -> List[Dict[str, Any]]:
-        """Returns all rooms to shutdown/purge. This includes those that has
+        """Returns all rooms to shutdown/purge. This includes those that have
         been interrupted by a stop/restart of synapse, but also scheduled ones
         like locally forgotten rooms.
         """