[media] DocBook media: correct description of reserved fields

Make sure that the documentation clearly states who is zeroing reserved
fields: drivers and/or applications.

This patch syncs the documentation with the checks that v4l2-compliance
and valgrind do.

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