[media] Documentation: add support for V4L touch devices

Document the new touch API.

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: fix up videodev2.h.rst.exceptions]

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>