[media] V4L2 Spec: fix extended control documentation

Update the spec to the behavior implemented by the control framework.
This should have been documented long ago but for some reason it was
never done.

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