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>

src/widgets/kernel/qwidget_qpa.cpp

@@ -879,9 +879,15 @@ int QWidget::metric(PaintDeviceMetric m) const
 }
 
 /*!
-    \preliminary
+    If this is a native widget, return the associated QWindow.
+    Otherwise return null.
 
-    Returns the QPlatformWindow this widget will be drawn into.
+    Native widgets include toplevel widgets, QGLWidget, and child widgets
+    on which winId() was called.
+
+    \since 5.0
+
+    \sa winId()
 */
 QWindow *QWidget::windowHandle() const
 {