Documentation: document ioctl interfaces better

Documentation: document ioctl interfaces better

Documentation/process/botching-up-ioctls.rst was orignally written as
a blog post for DRM driver writers, so it it misses some points while
going into a lot of detail on others.

Try to provide a replacement that addresses typical issues across a wider
range of subsystems, and follows the style of the core-api documentation
better.

Many improvements to the document are suggested by Ben Hutchings
<ben.hutchings@codethink.co.uk>, Jonathan Corbet <corbet@lwn.net> and
Geert Uytterhoeven <geert@linux-m68k.org>.

Reviewed-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>