GH-407: Clarify @KafkaListener Type Inference

Resolves #407

Add note to documentation.

Author: garyrussell

src/reference/asciidoc/kafka.adoc

@@ -729,6 +729,12 @@ public void jsonListener(Foo foo) {
 
 When using a `@KafkaListener`, the parameter type is provided to the message converter to assist with the conversion.
 
+[NOTE]
+====
+This type inference can only be achieved when the `@KafkaListener` annotation is declared at the method level.
+With a class-level `@KafkaListener`, the payload type is used to select which `@KafkaHandler` method to invoke so it must already have been converted before the method can be chosen.
+====
+
 NOTE: When using the `StringJsonMessageConverter`, you should use a `StringDeserializer` in the kafka consumer configuration and `StringSerializer` in the kafka producer configuration, when using Spring Integration or the `KafkaTemplate.send(Message<?> message)` method.
 
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