Move sub and sub-subtest documentation into their modules

Putting the docs. next to the code solves multiple problems:

*  Un-clutter main documentation
*  Help keep docs current by closer proximity to likely areas of change
*  Provide reporting-accessable human-readable description mapping to test names.
*  Allow for easier verification of general formatting in Travis CI.
`name_prefix` is widely used and can be in default section...

Actually we might consider either omitting default configs, or putting here list of all configs as references or list of default configs as references and explain the new ones. Each solution has it's drawbacks, IMO quite often people forbid to update it...