V4L/DVB: V4L Doc: correct the documentation for VIDIOC_QUERYMENU

The VIDIOC_QUERYMENU documentation was not correct. EINVAL can be returned
if the driver does not support some of the menu items. I.e. in a list of
MPEG bitrates a driver generally supports only a subset of these.

This behavior has been in place for years, but was never properly documented.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>