Doc: Describe editing the when condition of states

...in the Qt Quick Designer States pane.

Change-Id: Ie2a2252d295d6d0ae37744b7cc7691d5a1c3022f
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

doc/src/qtquick/qtquick-screens.qdoc

@@ -391,6 +391,18 @@
 
         \endlist
 
+    \section2 Using SCXML State Machines
+
+    To use QML and \QMLD together with an SCXML state machine, add states and
+    bind them to the state machine in the \uicontrol Backends tab, as described
+    in \l {Managing C++ Backend Objects}.
+
+    In the \uicontrol States pane, you can edit the \c when condition of states
+    to map QML states to the states of the SCXML state machine. For an example,
+    see \l {Qt SCXML Traffic Light QML Example (Dynamic)}.
+
+    \image qmldesigner-states-when-condition.png
+
     \section1 Animating Screens
 
     To make movement between states smooth, you can specify transitions. You can