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
79e4d1354d