docs: kerneldoc-preamble.sty: Expand comments in LaTeX code

Expand comments in LaTeX code and mention some of important points
told in changelogs of conf.py changes.
Hopefully they can help future contributors in this area.

No code change involved.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/bce9261b-1950-3146-07b2-07bd2ec79158@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>