xdg-shell/surface: Add note about window position and geometry

A client might want to change the window geometry without wanting the
window to be moved, for example when changing the width of the border.
Point out that the compositor should treat the (x,y) coordinate of the
geometry as the top-left corner of the window, and not change the
position of the window as it appears on the screen if the (x,y)
coordinate changes.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-By: Mike Blumenkrantz <zmike@osg.samsung.com>