Polish NestedConfigurationProperty documentation

Closes gh-9895

spring-boot-docs/src/main/asciidoc/appendix-configuration-metadata.adoc

@@ -815,6 +815,8 @@ Will produce meta-data information for `server.name`, `server.host.ip` and
 annotation on a field to indicate that a regular (non-inner) class should be treated as
 if it were nested.
 
+TIP: This has no effect on collections and maps as those types are automatically
+identified and a single meta-data property is generated for each of them.
 
 
 [[configuration-metadata-additional-metadata]]

spring-boot/src/main/java/org/springframework/boot/context/properties/NestedConfigurationProperty.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2014 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.
@@ -26,7 +26,10 @@ import java.lang.annotation.Target;
  * Indicates that a field in a {@link ConfigurationProperties} object should be treated as
  * if it were a nested type. This annotation has no bearing on the actual binding
  * processes, but it is used by the {@code spring-boot-configuration-processor} as a hint
- * that a field is not bound as a single value.
+ * that a field is not bound as a single value. When this is specified, a nested group is
+ * created for the field and its type is harvested.
+ * <p>
+ * This has no effect on collections and maps as these types are automatically identified.
  *
  * @author Stephane Nicoll
  * @author Phillip Webb