Expand on docstring comment about return value

synapse/storage/events.py

@@ -557,6 +557,11 @@ class EventsStore(EventsWorkerStore):
             Returns a tuple of two state maps, the first being the full new current
             state and the second being the delta to the existing current state.
             If both are None then there has been no change.
+
+            If there has been a change then we only return the delta if its
+            already been calculated. Conversely if we do know the delta then
+            the new current state is only returned if we've already calculated
+            it.
         """
 
         if not new_latest_event_ids: