https://www.mediawiki.org/wiki/Watchlist_Expiry is a great start, but there are still a few loose ends:
[x] Move [[ https://www.mediawiki.org/wiki/Watchlist_Expiry | Watchlist Expiry ]] to [[ https://www.mediawiki.org/wiki/Help:Watchlist_expiry | Help:Watchlist expiry ]] (all end user documentation belongs in the Help namespace and the page name should use sentence case rather than title case). Note that because the page is translatable, you will need to get a translation administrator (such as Johan) to perform the actual move.
[x] Add mention of watchlist expiry to [[ https://www.mediawiki.org/wiki/Help:Watchlist | Help:Watchlist ]] with appropriate links.
[x] Add mention of watchlist expiry to [[ https://www.mediawiki.org/wiki/Help:Watching_pages | Help:Watching pages ]] with appropriate links.
[] Add technical documentation for watchlist expiry to [[ https://www.mediawiki.org/wiki/Manual:Watchlist | Manual:Watchlist ]] (or a new Manual:Watchlist_expiry page).
Be sure to use `{{MW 1.35|+}}` where appropriate.