fix: java doc on MessageListener - acknowledgement part (#5429)

JavaDoc fix

MessageListener's message ack responsibility was not documented

pulsar-client-1x-base/pulsar-client-1x/src/main/java/org/apache/pulsar/client/api/MessageListener.java

@@ -33,6 +33,8 @@ public interface MessageListener<T> extends Serializable {
      *
      * This method will only be called once for each message, unless either application or broker crashes.
      *
+     * Implementation should acknowledge messages by calling consumer.acknowledge(msg).
+     *
      * Application is responsible of handling any exception that could be thrown while processing the message.
      *
      * @param consumer

pulsar-client-api/src/main/java/org/apache/pulsar/client/api/MessageListener.java

@@ -31,6 +31,8 @@ public interface MessageListener<T> extends Serializable {
      *
      * <p>This method will only be called once for each message, unless either application or broker crashes.
      *
+     * <p>Application is responsible for acking message by calling any of consumer acknowledgement methods.
+     *
      * <p>Application is responsible of handling any exception that could be thrown while processing the message.
      *
      * @param consumer