Update comment

synapse/storage/state.py

@@ -229,7 +229,8 @@ class StateStore(SQLBaseStore):
 
             # Prefill the state group cache with this group.
             # It's fine to use the sequence like this as the state group map
-            # is immutable.
+            # is immutable. (If the map wasn't immutable then this prefill could
+            # race with another update)
             txn.call_after(
                 self._state_group_cache.update,
                 self._state_group_cache.sequence,