Document @ConfigurationProperties on @Bean methods

Minor javadoc edits to hint that @ConfigurationProperties can also be
used with @Bean methods.

Fixes gh-1330

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

@@ -23,8 +23,9 @@ import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 /**
- * Annotation for externalized configuration. Add this to a class definition if you want
- * to bind and validate some external Properties (e.g. from a .properties file).
+ * Annotation for externalized configuration. Add this to a class definition or a
+ * {@code @Bean} method in a {@code @Configuration} class if you want to bind and validate
+ * some external Properties (e.g. from a .properties file).
  *
  * @author Dave Syer
  * @see ConfigurationPropertiesBindingPostProcessor