Documentation/kernel-docs.txt: some improvements on the ReST output

- Use lower case for sections, as this is the standard used on
  the other ReST files;
- The latest version of this document is at the Kernel source, and
  not at the listed URL. So, move it to the end of the doc.

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