Doc: fix documentation for QWidget::windowHandle.

It's not "preliminary" anymore, and it doesn't return a QPlatformWindow,
but a QWindow.

Change-Id: Iaa1938728e689d97011dce8cf0d88ff8cb4a83b9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>