[media] DocBook media: fix the poll() 'no QBUF' documentation

Clarify what poll() returns if STREAMON was called but not QBUF.
Make explicit the different behavior for this scenario for
capture and output devices.

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>