Missing documentation about migrating JGroups configuration

Migration Guide currently doesn't contain information about changes in JGroups subsystem configuration. At least the following changes should be described:

• the addition of <channels> element
• how to configure the default stack for a channel
• that the thread-factory attribute on the transport element is now deprecated and only usable in domains with EAP6 slaves
• that there are thread pools which are now used instead of thread-factory and references to the (now removed) threads subsystem.

The document should also link to Configuration Guide for further information on configuring JGroups.
Some more information on what has changed/what has been deprecated, can be gained by reading about the subsystem, or its parts, in CLI. E.g.:

/subsystem=jgroups/stack=tcp/transport=TCP:read-resource-description(recursive=true)