stable/viewporter: rephrase a wp_viewport paragraph

Cc: Yong Bakos <ybakos@humanoriented.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
[Pekka: s/culled/ignored/]
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>