[7.4.z] Documentation: add default values of JGroups

The values which are found in "jgroups-defaults.xml" should be documented.
Since these values might be changed when JGroups module is updated, they are required to be overwritten.

jgroups-defaults.xml can be found at
jboss-eap-7.3.5-src/jboss-eap-7.3-src/clustering/jgroups/extension/src/main/resources/jgroups-defaults.xml