[media] Docbook: media: Document changes on struct v4l2_ext_controls

Vidioc-g-ext-ctrls can now be used to get the default value of the
controls.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>