media: Clarify the meaning of file descriptors in VIDIOC_DQBUF

When the application calls VIDIOC_DQBUF with the DMABUF memory type, the
v4l2_buffer structure (or v4l2_plane structures) are filled with DMA-buf
file descriptors. However, the current documentation does not explain
whether those are new file descriptors referring to the same DMA-bufs or
just the same integers as passed to VIDIOC_QBUF back in time. Clarify
the documentation that it's the latter.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>