Update comment for new column

synapse/storage/schema/delta/56/current_state_events_membership.sql

@@ -16,6 +16,9 @@
 -- We add membership to current state so that we don't need to join against
 -- room_memberships, which can be surprisingly costly (we do such queries
 -- very frequently).
+-- This will be null for non-membership events and the content.membership key
+-- for membership events. (Will also be null for membership events until the
+-- background update job has finished).
 ALTER TABLE current_state_events ADD membership TEXT;
 
 INSERT INTO background_updates (update_name, progress_json) VALUES