GH-3400: Clarify manual ack semantics

Fixes #3400

spring-kafka-docs/src/main/antora/modules/ROOT/pages/kafka/receiving-messages/listener-annotation.adoc

@@ -173,7 +173,11 @@ If seekPosition set `TIMESTAMP`, `initialOffset` means timestamp.
 == Manual Acknowledgment
 
 When using manual `AckMode`, you can also provide the listener with the `Acknowledgment`.
+To activate the manual `AckMode`, you need to set the ack-mode in `ContainerProperties` to the appropriate manual mode.
 The following example also shows how to use a different container factory.
+This custom container factory must set the `AckMode` to a manual type by calling the `getContainerProperties()` and then calling `setAckMode` on it.
+Otherwise, the `Acknowledgment` object will be null.
+
 
 [source, java]
 ----

spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/MessagingMessageListenerAdapter.java

@@ -90,6 +90,7 @@
  * @author Nathan Xu
  * @author Wang ZhiYang
  * @author Huijin Hong
+ * @author Soby Chacko
  */
 public abstract class MessagingMessageListenerAdapter<K, V> implements ConsumerSeekAware, AsyncRepliesAware {
 
@@ -440,8 +441,7 @@ private RuntimeException checkAckArg(@Nullable Acknowledgment acknowledgment, Me
 		if (this.hasAckParameter && acknowledgment == null) {
 			return new ListenerExecutionFailedException("invokeHandler Failed",
 					new IllegalStateException("No Acknowledgment available as an argument, "
-							+ "the listener container must have a MANUAL AckMode to populate the Acknowledgment.",
-							ex));
+							+ "the listener container must have a MANUAL AckMode to populate the Acknowledgment."));
 		}
 		return new ListenerExecutionFailedException(createMessagingErrorMessage("Listener method could not " +
 				"be invoked with the incoming message", message.getPayload()), ex);