In the Migration Guide, 5.18. Application Clustering Changes, 5.18.1. Overview of New Clustering Features, it states:

JBoss EAP 7 introduces a new public API for building singleton services that significantly simplifies the process. For more information, see Implement an HA Singleton in Developing EJB Applications for JBoss EAP.

This topic is no longer there. We need to point them to the Development Guide (#clustered_ha_singleton_service):

For information on singletons, see HA Singleton Service in the JBoss EAP Development Guide.