Better generated configuration docs.

* Generates docs from more settings classes.
* Extracts information on valid values, deprecated or mandatory settings and default values.
* Makes every setting easy to link to.
* Uses new annotations @Internal and @Obsoleted to add information to settings.
* Cleans up language and formatting in settings descriptions.
* Upgrade of doctools, including moving to a JVM-based DocBook validation and better support for mobile devices.
Merge pull request #3380 from nawroth/docgen

Better generated configuration docs.