GH-3807: Doc about @KafkaHandler requirements

Fixes: #3807

Signed-off-by: chickenchickenlove <[email protected]>

Author: chickenchickenlove

spring-kafka-docs/src/main/antora/modules/ROOT/pages/kafka/receiving-messages/class-level-kafkalistener.adoc

@@ -2,6 +2,10 @@
 = `@KafkaListener` on a Class
 
 When you use `@KafkaListener` at the class-level, you must specify `@KafkaHandler` at the method level.
+If no `@KafkaHandler` on any methods of this class or its sub-classes, the framework will reject such a configuration.
+The `@KafkaHandler` annotation is required for explicit and concise purpose of the method.
+Otherwise it is hard to make a decision about this or other method without extra restrictions.
+
 When messages are delivered, the converted message payload type is used to determine which method to call.
 The following example shows how to do so:
 
@@ -58,4 +62,3 @@ void listen(Object in, @Header(KafkaHeaders.RECORD_METADATA) ConsumerRecordMetad
     ...
 }
 ----
-