Add mention of java.util.Optional in @MVC

Issue: SPR-12370

spring-web/src/main/java/org/springframework/web/bind/annotation/RequestMapping.java

@@ -160,6 +160,12 @@ import java.util.concurrent.Callable;
  * context path, and the literal part of the servlet mapping.
  * </ul>
  *
+ * <p><strong>Note: </strong> JDK 1.8's {@code java.util.Optional} is supported
+ * as a method parameter type with annotations that provide a {@code required}
+ * attribute (e.g. {@code @RequestParam}, {@code @RequestHeader}, etc.) The use
+ * of {@code java.util.Optional} in those cases is equivalent to having
+ * {@code required=false}.
+ *
  * <p>The following return types are supported for handler methods:
  * <ul>
  * <li>A {@code ModelAndView} object (Servlet MVC or Portlet MVC),