xdg-shell: Some minor clarifications

Mention set_window_geometry in configure documentation.

Add a strategic "For instance" to clarify what is just an example.

Clarify that the arguments of set_window_geometry are in the surface
local coordinate space.

Point out that the client needs to destroy a dismissed popup.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
1 file changed