Documentation/SubmitChecklist: update kernel-doc task

Task 11 (kernel-doc) still mentions usage of make manpages, but
this won't work if the API is documented via Sphinx. So, update
it to use either htmldocs or pdfdocs, with are the documentation
targets that work for all.

While here, add ReST reference to the kernel documentation book.

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