Add comments on min_stream_id

saying that the min stream id won't be completely accurate all the time

synapse/handlers/receipts.py

@@ -111,6 +111,7 @@ class ReceiptsHandler(BaseHandler):
             self.notifier.on_new_event(
                 "receipt_key", max_batch_id, rooms=affected_room_ids
             )
+            # Note that the min here shouldn't be relied upon to be accurate.
             self.hs.get_pusherpool().on_new_receipts(
                 min_batch_id, max_batch_id, affected_room_ids
             )

synapse/push/httppusher.py

@@ -87,6 +87,8 @@ class HttpPusher(object):
 
     @defer.inlineCallbacks
     def on_new_receipts(self, min_stream_id, max_stream_id):
+        # Note that the min here shouldn't be relied upon to be accurate.
+
         # We could check the receipts are actually m.read receipts here,
         # but currently that's the only type of receipt anyway...
         badge = yield push_tools.get_badge_count(self.hs, self.user_id)