GH-2895: Mention endpoint.setId() in the registration.adoc

Fixes: #2895

Since `id` is required for the `SimpleRabbitListenerEndpoint` definition,
it is better to show it in the docs sample and mention its importance.

src/reference/antora/modules/ROOT/pages/amqp/receiving-messages/async-annotation-driven/registration.adoc

@@ -14,6 +14,7 @@ public class AppConfig implements RabbitListenerConfigurer {
     @Override
     public void configureRabbitListeners(RabbitListenerEndpointRegistrar registrar) {
         SimpleRabbitListenerEndpoint endpoint = new SimpleRabbitListenerEndpoint();
+		endpoint.setId("someRabbitListenerEndpoint");
         endpoint.setQueueNames("anotherQueue");
         endpoint.setMessageListener(message -> {
             // processing
@@ -25,5 +26,7 @@ public class AppConfig implements RabbitListenerConfigurer {
 
 In the preceding example, we used `SimpleRabbitListenerEndpoint`, which provides the actual `MessageListener` to invoke, but you could just as well build your own endpoint variant to describe a custom invocation mechanism.
 
+NOTE: the `id` property is required for `SimpleRabbitListenerEndpoint` definition.
+
 It should be noted that you could just as well skip the use of `@RabbitListener` altogether and register your endpoints programmatically through `RabbitListenerConfigurer`.