[media] DocBook media: improve the poll() documentation

The poll documentation was incomplete: document how events (POLLPRI)
are handled and fix the documentation of what poll does for display devices
and streaming I/O.

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