media: buffer.rst: only set V4L2_BUF_FLAG_REQUEST_FD for QBUF

Document that V4L2_BUF_FLAG_REQUEST_FD should only be used with
VIDIOC_QBUF and cleared otherwise.

Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>