[media] DocBook media: rewrite frontend open/close

To fix the last xmllint errors the open and close function reference
description was rewritten based on the v4l2 open and close functions.

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