docs: kernel-doc.rst: add documentation about man pages

kernel-doc-nano-HOWTO.txt has a chapter about man pages
production. While we don't have a working  "make manpages"
target, add it.

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