MAN: Improve enumeration documentation
    
    Clarifies several issues about enumeration that should hopefully further
    decrease the usage of enumeration across sssd install base.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3486
    
    Reviewed-by: Justin Stephenson <jstephen@redhat.com>
    Reviewed-by: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>