Migration Guide: provide an appendix (appendices) with a reference of migration warnings for :migrate operations

The :migrate management operations can, as a part of the result, return a list of migration warnings. We should provide a reference of those warnings with an explanation, if such explanation exists. The reference should be in an appendix, otherwise the readability of the main text would suffer.

This has been asked for:

• messaging subsystem – https://issues.jboss.org/browse/JBEAP-1635?focusedCommentId=13163583#comment-13163583
• jacorb subsystem – https://issues.jboss.org/browse/JBEAP-2621?focusedCommentId=13165501#comment-13165501

As far as I'm aware, this hasn't been asked for the web subsystem yet, but we should have that too.