MAN: Document how the Global Catalog is used currently
    
    The existing documentation was outdated. Remove it and document what the
    current patchset adds.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3468
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>