MAN: Add sss-certmap man page regarding priority processing
    
    PR adds following text in PRIORITY section of man sss-certmap:
    The processing is stopped when a matched rule is found and no
    further rules are checked.
    
    Resolves: https://pagure.io/SSSD/sssd/issue/3469
    
    Reviewed-by: Justin Stephenson <jstephen@redhat.com>