Update comment

synapse/handlers/receipts.py

@@ -158,7 +158,8 @@ async def get_new_events_as(
         if from_key == to_key:
             return [], to_key
 
-        # We first need to fetch all new receipts
+        # Fetch all read receipts for all rooms, up to a limit of 100. This is ordered
+        # by most recent.
         rooms_to_events = await self.store.get_linearized_receipts_for_all_rooms(
             from_key=from_key, to_key=to_key
         )

synapse/storage/databases/main/receipts.py

@@ -278,7 +278,8 @@ def f(txn):
     async def get_linearized_receipts_for_all_rooms(
         self, to_key: int, from_key: Optional[int] = None
     ) -> Dict[str, JsonDict]:
-        """Get receipts for all rooms between two stream_ids.
+        """Get receipts for all rooms between two stream_ids, up
+        to a limit of the latest 100 read receipts.
 
         Args:
             to_key: Max stream id to fetch receipts upto.