Merge branch 'mauro-vars' into docs-mw

Mauro says:

As suggested and discussed with Randy, this small series add support
for documenting variables using kernel-doc.

- patch 1: add support for the new feature;
- patch 2: extends to support DEFINE_*;
- patch 3: document two media vars;
- patch 4: fix an issue on kernel-doc.rst markups and automarkup;
- patch 5: document it;
- patch 6: better handle DEFINE_ macros when they don't have static/type;

Since version 5, I'm using "c:macro" to describe variables, as it
avoids Sphinx C domain to try parse the variable. This makes it more
flexible and easier to maintain in long term.