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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>