[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>

Documentation/media/uapi/v4l/vidioc-g-dv-timings.rst

@@ -227,6 +227,14 @@ EBUSY
        -  Several flags giving more information about the format. See
 	  :ref:`dv-bt-flags` for a description of the flags.
 
+    -  .. row 17
+
+       -  __u32
+
+       -  ``reserved[14]``
+
+       -  Reserved for future extensions. Drivers and applications must set
+          the array to zero.
 
 
 .. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{7.0cm}|p{3.5cm}|