Add more information regarding where to find revocation reason in "ipa cert_revoke -h" and "ipa cert_find -h".
    
    According to review feedback, I changed the help message as follow
    
    $ ipa cert_revoke -h
    Usage: ipa [global-options] cert-revoke SERIAL-NUMBER [options]
    
    Revoke a certificate.
    Options:
      -h, --help            show this help message and exit
      --revocation-reason=INT
                            Reason for revoking the certificate (0-10). Type "ipa
                            help cert" for revocation reason details.
    
    https://fedorahosted.org/freeipa/ticket/5819
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Gabe Alford <redhatrises@gmail.com>