Warn about RSocket web server compatibility

This commit warns developers about the fact that plugging RSocket into
an existing web server is only possible with Reactor Netty web servers.
RSocket itself is using Reactor Netty, so this is why we can plug an
RSocket over websocket handler in an existing Reactor Netty handler.

This feature is not possible with other web servers, as existing APIs do
not make that possible.

Fixes gh-17494

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

@@ -2989,6 +2989,8 @@ In case of a WebFlux application (i.e. of type `WebApplicationType.REACTIVE`), t
 	#spring.rsocket.server.port= # no port is defined
 ----
 
+WARNING: Plugging RSocket into an existing web server is only supported with Reactor Netty, as RSocket itself is built with that library.
+
 The only other way to create an RSocket server is to start an independent, embedded RSocket server.
 Besides the dependency requirements, the only required configuration is to define a port for that server: