doc/sphinx: Track line-number of starting blocks

doc/sphinx: Track line-number of starting blocks

Design is pretty simple: kernel-doc inserts breadcrumbs with line
numbers, and sphinx picks them up. At first I went with a sphinx
comment, but inserting those at random places seriously upsets the
parser, and must be filtered. Hence why this version now uses "#define
LINEO " since one of these ever escape into output it's pretty clear
there is a bug.

It seems to work well, and at least the 2-3 errors where sphinx
complained about something that was not correct in kernel-doc text the
line numbers matched up perfectly.

v2: Instead of noodling around in the parser state machine, create
a ViewList and parse it ourselves. This seems to be the recommended
way, per Jani's suggestion.

v3:
- Split out ViewList pach. Splitting the kernel-doc changes from the
  sphinx ones isn't possible, since emitting the LINENO lines wreaks
  havoc with the rst formatting. We must filter them.

- Improve the regex per Jani's suggestions, and compile it just once
  for speed.

- Now that LINENO lines are eaten, also add them to function parameter
  descriptions. Much less content and offset than for in-line struct
  member descriptions, but still nice to know which exact continuation
  line upsets sphinx.

- Simplify/clarify the line +/-1 business a bit.

v4: Split out the scripts/kernel-doc changes and make line-numbers
opt-in, as suggested by Jani.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>