[media] vidioc-g-dv-timings.rst: document the v4l2_bt_timings reserved field

This field was never documented, and neither was it mentioned that
it should be zeroed by the application.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>