Document padlock flag

Author: jryans

docs/labs.md

@@ -93,3 +93,8 @@ tab as the single source of truth just yet.
 
 This adds a presence indicator in the room list next to DM rooms where the other
 person is online.
+
+## Show padlocks on invite only rooms (`feature_invite_only_padlocks`)
+
+This adds padlocks to room list tiles and room header for invite only rooms.
+This feature flag (unlike most) is enabled by default.