Document unified title and toolbar limitations.

Change-Id: Ib64a4586c186001730895e6771b01bb213d53b9f
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/widgets/widgets/qmainwindow.cpp

@@ -1500,6 +1500,13 @@ bool QMainWindow::event(QEvent *event)
 /*!
     \property QMainWindow::unifiedTitleAndToolBarOnMac
     \brief whether the window uses the unified title and toolbar look on Mac OS X
+
+    Note that the Qt 5 implementation has several limitations compared to Qt 4:
+    \list
+        \li Use in windows with OpenGL content is not supported. This includes QGLWidget and QOpenGLWidget.
+        \li Using dockable or movable toolbars may result in painting errors and is not recommended
+    \endlist
+
     \since 5.2
 */
 void QMainWindow::setUnifiedTitleAndToolBarOnMac(bool set)