Provide a working example instead of unclear placeholders

See gh-34828

Signed-off-by: Artur <artur@vaadin.com>

spring-webflux/src/main/java/org/springframework/web/reactive/socket/WebSocketHandler.java

@@ -45,13 +45,26 @@ import reactor.core.publisher.Mono;
  * 	public Mono<Void> handle(WebSocketSession session) {
  *
  * 		Flux<WebSocketMessage> output = session.receive()
- *			.doOnNext(message -> {
- * 				// ...
+ * 			.doOnNext(message -> {
+ * 				// This is for side effects such as
+ * 				// - Logging incoming messages
+ * 				// - Updating some metrics or counters
+ * 				// - Performing access checks or validations (non-blocking)
+ * 				System.out.println("Got message: " + message.getPayloadAsText());
  * 			})
  * 			.concatMap(message -> {
- * 				// ...
+ * 				// This is where you handle the actual processing of the incoming message. It
+ * 				// might involve:
+ * 				// - Parsing the message content (e.g., JSON parsing)
+ * 				// - Invoking a reactive service (e.g., database, HTTP call, etc.)
+ * 				// - Returning a transformed value, typically a Mono<String> or Mono<SomeType>
+ * 				// if you're mapping to another data format
+ * 				return Mono.just(message.getPayloadAsText());
  * 			})
- * 			.map(value -> session.textMessage("Echo " + value));
+ * 			.map(value -> {
+ * 				// This is where you produce one or more responses for the message
+ * 				return session.textMessage("Echo " + value));
+ * 			});
  *
  * 		return session.send(output);
  * 	}