Add an example for nested docking within MDI

src/DockWidgetBase.h

@@ -79,6 +79,7 @@ public:
         Option_DeleteOnClose = 4, ///< Deletes the DockWidget when closed
         Option_MDINestable = 8 ///< When this dock widget is being shown in a MDI area it will also allow other dock widgets to be dropped to its sides and tabbed
                                /// Usually Each MDI "window" corresponds to one DockWidget, with this option each "window" will have a layout with 1 or more dock widgets
+                               /// Run "kddockwidgets_mdi_with_docking_example -n" to see it in action
     };
     Q_DECLARE_FLAGS(Options, Option)
     Q_ENUM(Options);