Wayland: Add notes to doc that global positioning does not work on Wayland

Wayland by design does not support things like getting the global cursor position (QCursor::pos()) or explicitely positioning a window. It probably would be very helpful if either notes were added to all such methods that they will not work on Wayland, or if something like an overview of all of the unavailable methods were compiled somewhere (maybe as part of the already existing "Wayland and Qt" page).