MAN: Clarify the GC support a bit
    
    It should be noted that disabling GC does *not* disable lookups from
    trusted domains. Disabling GC might be a a good way for admins who wish
    to use POSIX attributes in trusted domains and the man page should hint
    this option.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    (cherry picked from commit fdaaf2525e333af04ee9b48429b6766b5fd6cab6)