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.
Yes, defaults should only be documented in ``index.rst``.  Unfortunately, ``name_prefix`` isn't yet a default, though it quite likely should be.  I think we can address this in a separate PR though.  I'll open an issue..