Expand docstring a bit

synapse/storage/events.py

@@ -1344,11 +1344,17 @@ class EventsStore(SQLBaseStore):
             self._get_event_cache.invalidate((event_id,))
 
     def _get_events_from_cache(self, events, allow_rejected, update_metrics=True):
-        """
+        """Fetch events from the caches
+
         Args:
             events (list(str)): list of event_ids to fetch
             allow_rejected (bool): Whether to teturn events that were rejected
             update_metrics (bool): Whether to update the cache hit ratio metrics
+
+        Returns:
+            dict of event_id -> _EventCacheEntry for each event_id in cache. If
+            allow_rejected is `False` then there will still be an entry but it
+            will be `None`
         """
         event_map = {}