[media] DocBook media: improve event documentation

It always annoyed me that the event type documentation was separate from the struct
v4l2_event documentation. This patch moves it all to one place, VIDIOC_DQEVENT.

This makes much more sense.

Also changed the 'changes-flags' ref to 'ctrl-changes-flags' since this referred to
control changes. There is a src-changes-flags as well, so 'changes-flags' was a bit
vague.

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