|Resolved||TTO||T12493 Setting a temporary usergroup (allow expiry of user rights via Special:UserRights form)|
|Resolved||TTO||T162517 Remove $wgDisableUserGroupExpiry feature flag|
|Resolved||TTO||T159416 [Tracking] Enable user group expiry on Wikimedia wikis|
|Resolved||kaldari||T161685 Document user group expiry|
The new docs don't say much about user group expiry, but I don't really think there is a whole lot to say.
I'm not sure that any documentation in the Manual: namespace is required. The feature is user-facing and doesn't really affect the wiki operators who are the target audience of Manual: space.