At the moment, that new feature is only documented on the sprint wiki page from the Community Tech team: https://meta.wikimedia.org/wiki/Community_Tech/Ping_users_from_the_edit_summary
It should be integrated to the central documentation concerning Notifications: https://www.mediawiki.org/wiki/Help:Notifications/Notifications_types#Mentions
A couple of sentences with minimal information to handle most cases would be enough. Theycan link to the meta page, Requirements section.
(Triaged as high because the feature has already been released.)