Update WebMvcConfigurer Javadoc for message converters

Closes gh-26388

spring-webflux/src/main/java/org/springframework/web/reactive/config/WebFluxConfigurer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -93,10 +93,13 @@ public interface WebFluxConfigurer {
 	}
 
 	/**
-	 * Configure custom HTTP message readers and writers or override built-in ones.
-	 * <p>The configured readers and writers will be used for both annotated
-	 * controllers and functional endpoints.
-	 * @param configurer the configurer to use
+	 * Configure the HTTP message readers and writers for reading from the
+	 * request body and for writing to the response body in annotated controllers
+	 * and functional endpoints.
+	 * <p>By default, all built-in readers and writers are configured as long as
+	 * the corresponding 3rd party libraries such Jackson JSON, JAXB2, and others
+	 * are present on the classpath.
+	 * @param configurer the configurer to customize readers and writers
 	 */
 	default void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
 	}