Document @WebFluxTest limitation with RouterFunction

Fixes gh-10683

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

@@ -6534,6 +6534,10 @@ example shows a class that uses both `@WebFluxTest` and a `WebTestClient`:
 A list of the auto-configuration that is enabled by `@WebFluxTest` can be
 <<appendix-test-auto-configuration#test-auto-configuration,found in the appendix>>.
 
+NOTE: `@WebFluxTest` cannot be used for testing routes registered via the functional
+web framework. For testing `RouterFunction` beans in the context, consider using
+`@SpringBootTest`.
+
 TIP: Sometimes writing Spring WebFlux tests is not enough; Spring Boot can help you run
 <<boot-features-testing-spring-boot-applications-testing-with-running-server,
 full end-to-end tests with an actual server>>.