Document how to parameterize REST Docs' output dir with WebTestClient

See gh-27755

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -8063,6 +8063,16 @@ If you require more control over Spring REST Docs configuration than offered by
 include::{code-examples}/test/autoconfigure/restdocs/webclient/AdvancedConfigurationExample.java[tag=configuration]
 ----
 
+If you want to make use of Spring REST Docs support for a parameterized output directory, you can create a `WebTestClientBuilderCustomizer` bean.
+The auto-configuration calls this `WebTestClientBuilderCustomizer` in order to create the `WebTestClient` bean for the current context
+with the consumer provided in the parameter of the method `entityExchangeResultConsumer`.
+The following example shows a `WebTestClientBuilderCustomizer` being defined:
+
+[source,java,indent=0,subs="verbatim"]
+----
+include::{code-examples}/test/autoconfigure/restdocs/webclient/MyWebTestClientBuilderCustomizerConfiguration.java[]
+----
+
 
 
 [[boot-features-testing-spring-boot-applications-testing-autoconfigured-rest-docs-rest-assured]]

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/test/autoconfigure/restdocs/webclient/MyWebTestClientBuilderCustomizerConfiguration.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2012-2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.features.testing.springbootapplications.autoconfiguredspringrestdocs.withwebtestclient;
+
+import org.springframework.boot.test.context.TestConfiguration;
+import org.springframework.boot.test.web.reactive.server.WebTestClientBuilderCustomizer;
+import org.springframework.context.annotation.Bean;
+
+import static org.springframework.restdocs.webtestclient.WebTestClientRestDocumentation.document;
+
+@TestConfiguration(proxyBeanMethods = false)
+public class MyWebTestClientBuilderCustomizerConfiguration {
+
+	@Bean
+	public WebTestClientBuilderCustomizer restDocumentation() {
+		return (builder) -> builder.entityExchangeResultConsumer(document("{method-name}"));
+	}
+
+}