Added JavaDoc for new constructor.

spring-integration-core/src/main/java/org/springframework/integration/message/GenericMessage.java

@@ -64,6 +64,15 @@ public class GenericMessage<T> implements Message<T> {
 		this.payload = payload;
 	}
 
+	/**
+	 * Create a new message with the given payload. The id will be generated by
+	 * the default {@link IdGenerator} strategy. The header will be populated
+	 * with the attributes and properties of the provided header.
+	 * 
+	 * @param payload the message payload
+	 * @param headerToCopy message header whose attributes and properties should
+	 * be copied into the new message's header
+	 */
 	public GenericMessage(T payload, MessageHeader headerToCopy) {
 		this(payload);
 		this.copyHeader(headerToCopy);