[Documentation] JGroups subsystem changed to allow better integration with Elytron

A commit was merged recently that refactors the JGroups subsystem and allows partial integration with the Elytron subsystem. The changes should be documented.

This is the PR: https://github.com/wildfly/wildfly/pull/9638/files
and these are the issues that the PR solves:

• https://issues.jboss.org/browse/WFLY-7882
• https://issues.jboss.org/browse/WFLY-8002

This is also one part of an RFE: https://issues.jboss.org/browse/EAP7-561

Good sources of knowledge:

• developers
• xsd differences between DR11 and DR12
• model differences between DR11 and DR12

From what I know, the main difference is that when before you'd add simple <protocol> elements to a JGroups protocol stack, now you can add <protocol>, <encrypt-protocol>, <jdbc-protocol> and a <socket-protocol> elements. Each of these expects a different set of properties.