Add doc for token in ResponseOptions #411

src/MQTTAsync.h

 /*******************************************************************************
- * Copyright (c) 2009, 2017 IBM Corp.
+ * Copyright (c) 2009, 2018 IBM Corp.
  *
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
@@ -461,7 +461,12 @@ typedef struct
     * provide access to the context information in the callback.
     */
 	void* context;
-	MQTTAsync_token token;   /* output */
+  /**
+    * A token is returned from the call.  It can be used to track
+    * the state of this request, both in the callbacks and in future calls
+    * such as ::MQTTAsync_waitForCompletion.
+    */
+	MQTTAsync_token token;
 } MQTTAsync_responseOptions;
 
 #define MQTTAsync_responseOptions_initializer { {'M', 'Q', 'T', 'R'}, 0, NULL, NULL, 0, 0 }