Document async requests with Spring MVC Test

Closes gh-19666

src/docs/asciidoc/testing.adoc

@@ -5115,6 +5115,61 @@ resulting links by using XPath expressions:
 ----
 ====
 
+
+[[spring-mvc-test-async-requests]]
+===== Async Requests
+
+Servlet 3.0 asynchronous requests,
+<<web.adoc#mvc-ann-async,supported in Spring MVC>>, work by exiting the Servlet container
+thread and allowing the application to compute the response asynchronously, after which
+an async dispatch is made to complete processing on a Servlet container thread.
+
+In Spring MVC Test, async requests can be tested by asserting the produced async value
+first, then manually performing the async dispatch, and finally verifying the response.
+Below is an example test for controller methods that return `DeferredResult`, `Callable`,
+or reactive type such as Reactor `Mono`:
+
+====
+[source,java,indent=0]
+[subs="verbatim,quotes"]
+----
+@Test
+public void test() throws Exception {
+	MvcResult mvcResult = this.mockMvc.perform(get("/path"))
+		.andExpect(status().isOk()) <1>
+		.andExpect(request().asyncStarted()) <2>
+		.andExpect(request().asyncResult("body")) <3>
+		.andReturn();
+
+	this.mockMvc.perform(asyncDispatch(mvcResult)) <4>
+		.andExpect(status().isOk()) <5>
+		.andExpect(content().string("body"));
+}
+----
+<1> Check response status is still unchanged
+<2> Async processing must have started
+<3> Wait and assert the async result
+<4> Manually perform an ASYNC dispatch (as there is no running container)
+<5> Verify the final response
+
+====
+
+
+
+
+[[spring-mvc-test-vs-streaming-response]]
+===== Streaming Responses
+
+There are no options built into Spring MVC Test for container-less testing of streaming
+responses. Applications that make use of
+<<web.adoc#mvc-ann-async-http-streaming,Spring MVC streaming>> options can use the
+<<testing.adoc#webtestclient-stream,WebTestClient>> to perform end-to-end, integration
+tests against a running server. This is also supported in Spring Boot where you can
+https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-testing-spring-boot-applications-testing-with-running-server[test a running server]
+with `WebTestClient`. One extra advantage is the ability to use the `StepVerifier` from
+project Reactor that allows declaring expectations on a stream of data.
+
+
 [[spring-mvc-test-server-filters]]
 ===== Filter Registrations
 
@@ -5133,26 +5188,13 @@ Registered filters are invoked through the `MockFilterChain` from `spring-test`,
 last filter delegates to the `DispatcherServlet`.
 
 
-[[spring-mvc-test-vs-streaming-response]]
-===== Streaming Responses
-
-There are no options built into Spring MVC Test for container-less testing of streaming
-responses. Applications that make use of
-<<web.adoc#mvc-ann-async-http-streaming,Spring MVC streaming>> options can use the
-<<testing.adoc#webtestclient-stream,WebTestClient>> to perform end-to-end, integration
-tests against a running server. This is also supported in Spring Boot where you can
-https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-testing-spring-boot-applications-testing-with-running-server[test a running server]
-with `WebTestClient`. One extra advantage is the ability to use the `StepVerifier` from
-project Reactor that allows declaring expectations on a stream of data.
-
-
 [[spring-mvc-test-vs-end-to-end-integration-tests]]
-===== Differences Between Out-of-Container and End-to-End Integration Tests
+===== Spring MVC Test vs End-to-End Tests
 
-As mentioned earlier Spring MVC Test is built on the Servlet API mock objects from the
+Spring MVC Test is built on Servlet API mock implementations from the
-`spring-test` module and does not use a running Servlet container. Therefore, there are
+`spring-test` module and does not rely on a running container. Therefore, there are
-some important differences compared to full end-to-end integration tests with an actual
+some differences when compared to full end-to-end integration tests with an actual
-client and server running.
+client and a live server running.
 
 The easiest way to think about this is by starting with a blank `MockHttpServletRequest`.
 Whatever you add to it is what the request becomes. Things that may catch you by surprise
@@ -5193,7 +5235,7 @@ important thing to check. In short, there is room here for multiple styles and s
 of testing even within the same project.
 
 [[spring-mvc-test-server-resources]]
-===== Further Server-Side Test Examples
+===== Further Examples
 
 The framework's own tests include
 https://github.com/spring-projects/spring-framework/tree/master/spring-test/src/test/java/org/springframework/test/web/servlet/samples[many