Add a comment explaining why the filter cache doesn't need exipiring

synapse/replication/slave/storage/filtering.py

@@ -21,4 +21,5 @@ class SlavedFilteringStore(BaseSlavedStore):
     def __init__(self, db_conn, hs):
         super(SlavedFilteringStore, self).__init__(db_conn, hs)
 
+    # Filters are immutable so this cache doesn't need to be expired
     get_user_filter = FilteringStore.__dict__["get_user_filter"]