Update pam_sss.8.xml
    
    pam_sss: Added return values on a man page
    
    Resolves: https://pagure.io/SSSD/sssd/issue/3672
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>