scripts/kernel-doc: Add option to inject line numbers

Opt-in since this wreaks the rst output and must be removed
by consumers again. This is useful to adjust the linenumbers
for included kernel-doc snippets in shinx. With that sphinx
error message will be accurate when there's issues with the
rst-ness of the kernel-doc comments.

Especially when transitioning a new docbook .tmpl to .rst this
is extremely useful, since you can just use your editors compilation
quickfix list to accurately jump from error to error.

v2:
- Also make sure that we filter the LINENO for purpose/at declaration
  start so it only shows for selected blocks, not all of them (Jani).
  While at it make it a notch more accurate.
- Avoid undefined $lineno issues. I tried filtering these out at the
  callsite, but Jani spotted more when linting the entire kernel.
  Unamed unions and similar things aren't stored consistently and end
  up with an undefined line number (but also no kernel-doc text, just
  the parameter type). Simplify things and filter undefined line
  numbers in print_lineno() to catch them all.

v3: Fix LINENO 0 issue for kernel-doc comments without @param: lines
or any other special sections that directly jump to the description
after the "name - purpose" line. Only really possible for functions
without parameters. Noticed 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>