[media] v4l2-event.h: add overview documentation to the header

It's getting confusing who is linking to what, so add an overview at
the start of the header.

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