TW: Update docs to add `User Cap` setting details

Technical Writing issue for the new "Max User" setting described in &4315 (closed). We should add the information to the https://docs.gitlab.com/ee/subscriptions docs.

As noted in this comment we should consider making this docs update here: https://docs.gitlab.com/ee/user/admin_area/settings/sign_up_restrictions.html

Update: per this comment we are using User Cap for this feature.