[media] DocBook media: document new motion detection controls

Document the 'Detect' control class and the new Motion Detection controls.
Those controls will be used by the solo6x10 and go7007 drivers.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>