The "Migrate Clustering HA Singleton" topic contains the following information:

For more information about how to implement an HA singleton, see Implement an HA Singleton in Developing EJB Applications for JBoss EAP.

That content was moved to the Development Guide here: https://access.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/development-guide/#clustered_ha_singleton_service

https://access.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/migration-guide/#migrate_clustering_ha_singleton