MAN: Describe the constrains of ipa_server_mode better in the man page
    
    Amends the sssd-ipa man page so that we explicitly say that:
        * SSSD needs to be pointed at the IPA server itself
        * SSSD currently needs to print fully qualified names for users for
          trusted domains.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3484
    
    Reviewed-by: Justin Stephenson <jstephen@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>