Improve discoverability of MqttCallbackExtended

It's really hard to find the interface if you don't already know about it.
Nothing points to it, yet it's mandatory to subscribe to topics in the
method only available in this interface, otherwise the client will not
subscribe back to the topics after an automatic reconnection.

Signed-off-by: bendem <online@bendem.be>

Author: bendem

org.eclipse.paho.client.mqttv3/src/main/java/org/eclipse/paho/client/mqttv3/IMqttClient.java

@@ -847,6 +847,7 @@ public interface IMqttClient extends AutoCloseable { //extends IMqttAsyncClient
 	 * as connect and subscribe can be tracked using the {@link MqttToken} passed to the
 	 * operation<p>
 	 * @see MqttCallback
+	 * @see MqttCallbackExtended
 	 * @param callback the class to callback when for events related to the client
 	 */
 	public void setCallback(MqttCallback callback);

org.eclipse.paho.client.mqttv3/src/main/java/org/eclipse/paho/client/mqttv3/MqttCallback.java

@@ -22,6 +22,7 @@
  * Classes implementing this interface
  * can be registered on both types of client: {@link IMqttClient#setCallback(MqttCallback)}
  * and {@link IMqttAsyncClient#setCallback(MqttCallback)}
+ * @see MqttCallbackExtended
  */
 public interface MqttCallback {
 	/**