documented that .shake() is iOS only

Author: Jonahss

src/main/java/io/appium/java_client/AppiumDriver.java

@@ -441,6 +441,7 @@ public void lockScreen(int seconds) {
 
   /**
    * Simulate shaking the device
+   * This is an iOS-only method
    */
   public void shake() {
     execute(SHAKE);