Doc: add a note about the tickmarksEnabled property

Change-Id: I61e6029c813fd0258a24c6ea547190808f42edc9
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>

src/controls/Slider.qml

@@ -167,6 +167,8 @@ Control {
         \l stepSize property.
 
         The default value is \c false.
+
+        \note This property may be ignored on some platforms when using the native style (e.g. Android).
     */
     property bool tickmarksEnabled: false