Document that QWindow::setFramePosition() and QWindow::setGeometry() can cause the window to be moved asynchronously

In QTBUG-73128 it was discovered that QWindow::setFramePosition() (or rather a function that calls it) was not given enough time to wait for the actual window position to update, because the move can happen asynchronously.

Having this fact stated in the documentation would have made diagnosing that issue easier, not to mention that this is a public function, so users could conceivably use it for their auto tests too.