Document Message#annotate(String,String) and Message#copy()

References #379

Author: acogoluegnes

src/main/java/com/rabbitmq/stream/Message.java

@@ -86,11 +86,29 @@ public interface Message {
    */
   Map<String, Object> getMessageAnnotations();
 
+  /**
+   * Add a message annotation to the message.
+   *
+   * @param key the message annotation key
+   * @param value the message annotation value
+   * @return the modified message
+   * @since 0.12.0
+   */
   default Message annotate(String key, Object value) {
     this.getMessageAnnotations().put(key, value);
     return this;
   }
 
+  /**
+   * Create a copy of the message.
+   *
+   * <p>The message copy contains the exact same instances of the original bare message (body,
+   * properties, application properties), only the message annotations are actually copied and can
+   * be modified independently.
+   *
+   * @return the message copy
+   * @since 0.12.0
+   */
   default Message copy() {
     return this;
   }