[ticket/12435] purge_notifications() fails for unused notifications

https://tracker.phpbb.com/browse/PHPBB3-12435

PHPBB3-12435
[ticket/12435] Update the comment

PHPBB3-12435
Merge pull request #2456 from Nicofuma/ticket/12435

[ticket/12435] purge_notifications() fails for unused notifications

* Nicofuma/ticket/12435:
  [ticket/12435] Update the comment
  [ticket/12435] purge_notifications() fails for unused notifications