tests: realmdomains_plugin: Add explanatory comment
    
    The realmdomains_mod command will fail if the testing environment
    is configured improperly and the IPA domain's NS/SOA records are
    not resolvable. This can easily happen if the machine's DNS server
    is not configured to the IPA server.
    
    Leave a explanatory note in the class.
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>