Add some docstrings (and avoid a duplicate query) in push actions processing

synapse/storage/databases/main/event_push_actions.py

@@ -910,6 +910,8 @@ def _handle_new_receipts_for_notifs_txn(self, txn: LoggingTransaction) -> bool:
         # recalculate the summary.
         for _, room_id, user_id, stream_ordering in rows:
             # Only handle our own read receipts.
+            #
+            # XXX Is this needed, the query filters these out?
             if not self.hs.is_mine_id(user_id):
                 continue