media: docs: vidioc-dqbuf: State all remaining fields are filled by driver

The specification for VIDIOC_DQBUF is slightly ambiguous on what fields
of |struct v4l2_buffer| is filled by the driver.

Reword it so things are clear: the driver fills in all remaining fields
not specified to be filled in by userspace applications.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>