Mention DEFAULT_NONE in @RequestMapping's Javadoc

This change mentions `ValueConstants.DEFAULT_NONE` in @RequestMapping's
Javadoc, in order to explain the default value.

Issue: SPR-9967

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

@@ -288,6 +288,7 @@ public @interface RequestMapping {
 	 * <p><b>Supported at the type level as well as at the method level!</b>
 	 * When used at the type level, all method-level mappings inherit
 	 * this primary mapping, narrowing it for a specific handler method.
+	 * @see org.springframework.web.bind.annotation.ValueConstants.DEFAULT_NONE
 	 */
 	String[] value() default {};