War deployment is not supported for WebFlux apps

Even though War deployment is supported by Spring Framework itself for
WebFlux applications, this is not the case currently in Spring Boot.

Fixes gh-12455

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

@@ -2842,6 +2842,10 @@ This section answers common questions about traditional deployment.
 [[howto-create-a-deployable-war-file]]
 === Create a Deployable War File
 
+WARNING: Because Spring WebFlux does not strictly depend on the Servlet API and
+applications are deployed by default on an embedded Reactor Netty server,
+War deployment is not supported for WebFlux applications.
+
 The first step in producing a deployable war file is to provide a
 `SpringBootServletInitializer` subclass and override its `configure` method. Doing so
 makes use of Spring Framework's Servlet 3.0 support and lets you configure your