[EO12] Migration Guide: content about migrating authenticator valves

The Migration Guide has a nice content on migrating custom valves, but the chapter 5.13.2. Migrate Custom Authenticator Valves that speaks about authenticator valves in inadequate.

I'm thinking about two things:

• EAP 6 shipped a bunch of custom valves that could be used for authentication. We should have a list of those and for each, provide a link to 7.0 documentation on how an equivalent is achieved in 7.0. We already have one of those, but that isn't all.
  • CC'ing rhatlapa@redhat.com and jcacek, as they have much better idea who to ask about that.
• Also, in EAP 6, customers were able to create completely custom authenticator valves (typically extending AuthenticatorBase or a subclass of that; AuthenticatorBase itself extends ValveBase). I have no idea if this was actually supported and if customers used it. If so, we should have a documentation on creating custom AuthenticationMechanism-s. That wouldn't of course belong to Migration Guide; the Migration Guide would only link to it. But we first have to figure out if it is a real migration concern or not.
  • sgilda_jira, could you please help figuring out if this is real? I guess you will also know better than me who to ask
  • If yes, I will create a Dev Guide (or something) JIRA. The basic steps are fairly simple (see https://issues.jboss.org/browse/JBEAP-4197?focusedCommentId=13194402&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13194402), but I'm not sure if we want to document this unless there are known usecases.