[7.1] Migration Tool Doc - Suggested warning to add to migration tool

The customer upgraded the domain controller from 7.0 to 7.1 before upgrading the hosts.

While that may be technically true, the customer did not realize that the new subsystems added by the JBoss Server Migration Tool would cause a 7.0 host to fail to boot.

Though he sees documentation about how to work around this in the Configuration Guide [1], he thinks it would be helpful to add a warning in Using the JBoss Server Migration Tool.

Specifically, he thinks a warning box at the top of [2] would be helpful, explaining that users cannot just run the tool on the domain controller and expect the other hosts to still work (i.e. unless users follow [1]).

[1] https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.1/html/configuration_guide/domain_management#configure_eap71_administer_eap70

[2] https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.1/html/using_the_jboss_server_migration_tool/migrating_jboss_eap_7_0_configurations_to_jboss_eap_7_1#migrating_a_jboss_eap_7_0_managed_domain_to_jboss_eap_7_1