Mention how to configure the web application type in tests

Closes gh-11025

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

@@ -5990,6 +5990,23 @@ TIP: Do not forget to add `@RunWith(SpringRunner.class)` to your test. Otherwise
 annotations are ignored.
 
 
+==== Detecting Web Application Type
+If Spring MVC is available, a regular MVC-based application context is configured. If you
+have only Spring WebFlux, we'll detect that and configure a WebFlux-based application
+context instead.
+
+If both are present, Spring MVC takes precedence. If you want to test a reactive web
+application in this scenario, you must set the `spring.main.web-application-type`
+property:
+
+[source,java,indent=0]
+----
+	@RunWith(SpringRunner.class)
+	@SpringBootTest(properties = "spring.main.web-application-type=reactive")
+	public class MyWebFluxTests { ... }
+----
+
+
 
 [[boot-features-testing-spring-boot-applications-detecting-config]]
 ==== Detecting Test Configuration