docs: doc-guide: parse-headers.rst update its documentation

docs: doc-guide: parse-headers.rst update its documentation

Besides converting from Perl to Python, parse-headers has gained
some new functionality and was moved to tools/docs.

Update its documentation accordingly.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/linux-doc/9391a0f0-7c92-42aa-8190-28255b22e131@infradead.org/
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <1f9025dc98dc58da3cc31f3343d5027f351be338.1762594622.git.mchehab+huawei@kernel.org>