[media] v4l2-event.h: fix comments and add to DocBook

The comments there are good enough for DocBook, however they're
using a wrong format. Fix and add to device-drivers.xml.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>