docs: retitle the kernel-documentation.rst

Let's make the title of this document (which shows up in the top page)
better describe its contents.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-documentation.rst

-==========================
-Linux Kernel Documentation
-==========================
+=================================
+How to write kernel documentation
+=================================
 
 Introduction
 ============