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.
Actually I think we should explicitly put 1.,2.,3., here, because people might want to see it in source. The output would be the same, but the one who will update it will be more careful.