Link to topLevelChanged() in the docs of QDockWidget::floating

topLevelChanged() is emitted when the floating property changes.
It's not very well named, it's easy to miss.

Change-Id: Iabaa4fb3dc6190df43d719ed7565f0586816c6de
Reviewed-by: Martin Smith <martin.smith@theqtcompany.com>

src/widgets/widgets/qdockwidget.cpp

@@ -1311,7 +1311,9 @@ QDockWidget::DockWidgetFeatures QDockWidget::features() const
 
     By default, this property is \c true.
 
-    \sa isWindow()
+    When this property changes, the \c {topLevelChanged()} signal is emitted.
+
+    \sa isWindow(), topLevelChanged()
 */
 void QDockWidget::setFloating(bool floating)
 {