Document that we auto-block AD users
Document that in both CE and EE, we will auto-block GitLab users when the same user is disabled in Active Directory.
We don't currently mention this in the docs, but it's useful to know. The specifics are that we will auto-block any user that has userAccountControl:1.2.840.113556.1.4.803
= 2, which is apparently 'disabled' in AD speak.