Merge branch 'sphinx-for-docs-next' into doc/4.8

Jani Nikula says:

Jon, this is v2 of [1] and [2], with a considerable amount of polish and
fixes added. We started dogfooding this within drm-intel, and Daniel has
reviewed the lot and contributed a number of fixes, most notably
accurate file and line number references from Sphinx build
errors/warnings to the kernel-doc comments in source code.

We believe this is now in good shape for merging for v4.8. It's all in
my sphinx-for-docs-next branch that you've already looked at; pull
details below.

When this lands in docs-next and we can backmerge to drm, we'll plunge
ahead and convert gpu.tmpl to rst, and have that ready for v4.8. We
think it's best to contribute that via the drm tree, as it'll involve
splitting up the documentation and likely numerous updates to kernel-doc
comments.

I plan to update Documentation/kernel-doc-nano-HOWTO.txt for Sphinx and
rst, obviously converting it to rst while at it.