Document the purpose of the wildflyee.api module in its module.xml, and mark it as private

The wildflyee.api module's use is fairly obscure and easy to misunderstand. And easy to forget, leading to wasting time every couple of years when we need to deal with it in some way. So add xml comments to it's module.xml documenting its purpose.

Also, add a jboss-api=private entry, preceded by an explanatory comment. Deployments should not directly depend on this module and if they do they should get the log WARN that comes with a dep on a private module.

Static modules provided by end users can depend on this module, and may do so as a way to avoid spelling out all their module's deps explicitly. But that's not good practice, so marking it as private with a comment may help warn such users away from doing that, if they happen to look.

WF's own internal use of the module is not affected by marking it private.