[media] DocBook media: make explicit that standard/timings never change automatically

A driver might detect a new standard or DV timings, but it will never change to
those new timings automatically. Instead it will send an event and let the application
take care of it.

Make this explicit in the documentation.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>