diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index 27284cd08..7cb314dee 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -306,6 +306,12 @@ class EventFederationStore(SQLBaseStore):
         self._update_extremeties(txn, events)
 
     def _update_extremeties(self, txn, events):
+        """Updates the event_*_extremities tables based on the new/updated
+        events being persisted.
+
+        This is called for new events *and* for events that were outliers, but
+        are are now being persisted as non-outliers.
+        """
         events_by_room = {}
         for ev in events:
             events_by_room.setdefault(ev.room_id, []).append(ev)