Android: add note on usage of QtNative.runAction()

Task-number: QTBUG-118077
Change-Id: I2a743c1b65733e1ca0bf20d172d35c46d7ec8b08
Reviewed-by: Tinja Paavoseppä <tinja.paavoseppa@qt.io>

src/android/jar/src/org/qtproject/qt/android/QtNative.java

@@ -202,6 +202,8 @@ public class QtNative
         updateApplicationState(state);
     }
 
+    // Post a runnable to Main (UI) Thread if the app is active,
+    // otherwise, queue it to be posted when the the app is active again
     public static void runAction(Runnable action)
     {
         synchronized (m_mainActivityMutex) {