ivi-layout: clarify get_layers_under_surface doc

This is derived from the implementation. I was not sure whether "under"
referred to object relationships or region intersections.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emre Ucan <eucan@de.adit-jv.com>
1 file changed