Provide test auto-configuration for Spring REST Docs

This commit introduces a new annotation, @AutoConfigureRestDocs,
which can be used to enable auto-configuration of Spring REST Docs.
The auto-configuration removes the need to use Spring REST Docs' JUnit
rule and will automatically configure MockMvc. Combined with the new
auto-configuration for MockMvc it allows a test class to be free of
boilerplate configuration:

@RunWith(SpringRunner.class)
@WebMvcTest
@AutoConfigureRestDocs(outputDir = "target/generated-snippets",
        uriScheme = "https", uriHost = "api.example.com",
        uriPort = 443)
public class ExampleDocumentationTests {

    @Autowired
    private MockMvc mvc;

    @Test
    public void documentIndex() {
        // …
    }

}

For more advanced customization a RestDocsMockMvcConfigurationCustomizer
bean can be used.

If a RestDocumentationResultHandler is found in the context, it will
be passed to the ConfigurableMockMvcBuilder's alwaysDo method as part
of its customization.

Closes gh-5563

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

@@ -4744,6 +4744,84 @@ database you can use the `@AutoConfigureTestDatabase` annotation:
 
 
 
+[[boot-features-testing-spring-boot-applications-testing-autoconfigurd-rest-docs]]
+==== Auto-configured Spring REST Docs tests
+Test `@AutoConfigureRestDocs` annotation can be used if you want to use Spring REST Docs
+in your tests. It will automatically configure `MockMvc` to use Spring REST Docs and
+removes the need for Spring REST Docs' JUnit rule.
+
+[source,java,indent=0]
+----
+	import org.junit.Test;
+	import org.junit.runner.RunWith;
+
+	import org.springframework.beans.factory.annotation.Autowired;
+	import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
+	import org.springframework.http.MediaType;
+	import org.springframework.test.context.junit4.SpringRunner;
+	import org.springframework.test.web.servlet.MockMvc;
+
+	import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.document;
+	import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+	import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
+
+	@RunWith(SpringRunner.class)
+	@WebMvcTest(UserController.class)
+	@AutoConfigureRestDocs("target/generated-snippets")
+	public class UserDocumentationTests {
+
+		@Autowired
+		private MockMvc mvc;
+
+		@Test
+		public void listUsers() throws Exception {
+			this.mvc.perform(get("/users").accept(MediaType.TEXT_PLAIN))
+					.andExpect(status().isOk())
+					.andDo(document("list-users"));
+		}
+
+	}
+----
+
+In addition to configuring the output directory, `@AutoConfigureRestDocs` can also
+configure the host, scheme, and port that will appear in any documented URIs. If you
+require more control over Spring REST Docs' configuration a
+`RestDocsMockMvcConfigurationCustomizer` bean can be used:
+
+[source,java,indent=0]
+----
+	@TestConfiguration
+	static class CustomizationConfiguration
+			implements RestDocsMockMvcConfigurationCustomizer {
+
+		@Override
+		public void customize(MockMvcRestDocumentationConfigurer configurer) {
+			configurer.snippets().withTemplateFormat(TemplateFormats.markdown());
+		}
+
+	}
+----
+
+If you want to make use of Spring REST Docs' support for a parameterized output directory,
+you can create a `RestDocumentationResultHandler` bean. The auto-configuration will
+call `alwaysDo` with this result handler, thereby causing each `MockMvc` call to
+automatically generate the default snippets:
+
+[source,java,indent=0]
+----
+	@TestConfiguration
+	static class ResultHandlerConfiguration{
+
+		@Bean
+		public RestDocumentationResultHandler restDocumentation() {
+			return MockMvcRestDocumentation.document("{method-name}");
+		}
+
+	}
+----
+
+
+
 [[boot-features-testing-spring-boot-applications-with-spock]]
 ==== Using Spock to test Spring Boot applications
 If you wish to use Spock to test a Spring Boot application you should add a dependency