docs-rst: parse-headers.pl: cleanup the documentation

Keeping both rst and in-file documentation in sync can be harsh.

So, simplify the script's internal documntation to a bare minimum,
and add a mention to the ReST file with its full documentation.

This way, a quick help is still available at the command line,
while the complete one is maintained at the ReST format.

As we won't be using pad2rst anymore, do a cleanup at the ReST
file.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>