Wildfly 11 is around the corner, your wildfly admin guide can be about Wilddly 10 and the new features en Wildfly 11.
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documentation is getting a major overhaul in very near future.
We are moving of confluence Documentation - Latest WildFly Documentation - Project Documentation Editor
to asciidoc based documentation that will be part of our source repo.
Current progress can be found at GitHub - ctomc/docs-playground
As for missing topics go, question is more of what part you are most familiar with.
I think that if we would have better "getting started" guide, especially the basic stuff how to install the server, how to setup JDK.
enviroment variables and how to install it as a service on Linux and on Windows.
this last part is something we are really missing.
you can also ping me on our hipchat channel about more info.