Document media type registration for appcache files

Since appcache manifests can have various file extensions, developers
should register the (file extension, media type) mapping in their Spring
MVC / Reactive Web configuration.

This commit adds javadoc on both `AppCacheManifestTransformer` variants
to explain how to do that.

Issue: SPR-14510

spring-web-reactive/src/main/java/org/springframework/web/reactive/resource/AppCacheManifestTransformer.java

@@ -53,13 +53,16 @@ import org.springframework.web.server.ServerWebExchange;
  * of the manifest in order to trigger an appcache reload in the browser.
  * </ul>
  *
- * All files that have the ".appcache" file extension, or the extension given
- * in the constructor, will be transformed by this class.
- *
- * <p>This hash is computed using the content of the appcache manifest and the
- * content of the linked resources; so changing a resource linked in the manifest
+ * <p>All files that have the ".appcache" file extension, or the extension given in the constructor,
+ * will be transformed by this class. This hash is computed using the content of the appcache manifest
+ * and the content of the linked resources; so changing a resource linked in the manifest
  * or the manifest itself should invalidate the browser cache.
  *
+ * <p>In order to serve manifest files with the proper {@code "text/manifest"} content type,
+ * it is required to configure it with
+ * {@code requestedContentTypeResolverBuilder.mediaType("appcache", MediaType.valueOf("text/manifest")}
+ * in {@code WebReactiveConfiguration.configureRequestedContentTypeResolver()}.
+ *
  * @author Rossen Stoyanchev
  * @author Brian Clozel
  * @since 5.0

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/AppCacheManifestTransformer.java

@@ -47,11 +47,15 @@ import org.springframework.util.StringUtils;
  * thus changing the content of the manifest in order to trigger an appcache reload in the browser.
  * </ul>
  *
- * All files that have the ".appcache" file extension, or the extension given in the constructor,
- * will be transformed by this class.
+ * <p>All files that have the ".appcache" file extension, or the extension given in the constructor,
+ * will be transformed by this class. This hash is computed using the content of the appcache manifest
+ * and the content of the linked resources; so changing a resource linked in the manifest
+ * or the manifest itself should invalidate the browser cache.
  *
- * <p>This hash is computed using the content of the appcache manifest and the content of the linked resources;
- * so changing a resource linked in the manifest or the manifest itself should invalidate the browser cache.
+ * <p>In order to serve manifest files with the proper {@code "text/manifest"} content type,
+ * it is required to configure it with
+ * {@code contentNegotiationConfigurer.mediaType("appcache", MediaType.valueOf("text/manifest")}
+ * in a {@code WebMvcConfigurerAdapter}.
  *
  * @author Brian Clozel
  * @since 4.1