SendTo Doc Polishing

- explicitly mention the needed reply template.

Author: garyrussell

src/reference/asciidoc/kafka.adoc

@@ -1595,6 +1595,10 @@ public class MultiListenerSendTo {
 ----
 ====
 
+IMPORTANT: In order to support `@SendTo`, the listener container factory must be provided with a `KafkaTemplate` (in its `replyTemplate` property), which is used to send the reply.
+This should be a `KafkaTemplate` and not a `ReplyingKafkaTemplate` which is used on the client-side for request/reply processing.
+When using Spring Boot, boot will auto-configure the template into the factory; when configuring your own factory, it must be set as shown in the examples below.
+
 Starting with version 2.2, you can add a `ReplyHeadersConfigurer` to the listener container factory.
 This is consulted to determine which headers you want to set in the reply message.
 The following example shows how to add a `ReplyHeadersConfigurer`: