Clarify behaviour of SpringApplicationBuilder.sibling(Object...)

Closes gh-8898

spring-boot/src/main/java/org/springframework/boot/builder/SpringApplicationBuilder.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -239,7 +239,10 @@ public class SpringApplicationBuilder {
 
 	/**
 	 * Create a sibling application (one with the same parent). A side effect of calling
-	 * this method is that the current application (and its parent) are started.
+	 * this method is that the current application (and its parent) are started without
+	 * any arguments if they are not already running. To supply arguments when starting
+	 * the current application and its parent use {@link #sibling(Object[], String...)}
+	 * instead.
 	 * @param sources the sources for the application (Spring configuration)
 	 * @return the new sibling builder
 	 */