Refine Forwarded headers configuration for Reactor servers

This commit introduces new properties for configuring the HTTP headers
that should be used for forward support.
The new `server.netty.forwarded-headers.header-format` property allows
"standard" or "x_forwarded" to select "Forwarded" or "X-Forwarded-*"
headers.

This aligns the behavior of Netty server with the "FRAMEWORK" strategy.

Closes gh-51031

documentation/spring-boot-docs/src/docs/antora/modules/how-to/pages/webserver.adoc

@@ -521,8 +521,8 @@ If your web server of choice supports the set of HTTP headers you need, setting
 | xref:how-to:webserver.adoc#howto.webserver.use-behind-a-proxy-server.jetty[]
 
 | Reactor Netty
-| `"X-Forwarded-*"`
-|
+| `"X-Forwarded-*"`, `"Forwarded"`
+| xref:how-to:webserver.adoc#howto.webserver.use-behind-a-proxy-server.reactor-netty[]
 
 |===
 
@@ -587,6 +587,24 @@ For more specific options, you can switch off the forwarded configuration (using
 
 
 
+[[howto.webserver.use-behind-a-proxy-server.reactor-netty]]
+=== Customize Reactor Netty's Proxy Configuration
+
+If you use Reactor Netty, you can additionally configure the type of the headers used to carry "`forwarded`" information.
+By default, the `"X-Forwarded-*"` format is used, but you can opt into the standard RFC variant:
+
+[configprops,yaml]
+----
+server:
+  netty:
+    forwarded-headers:
+      header-format: "standard"
+----
+
+For more specific options, you can switch off the forwarded configuration (using `server.forward-headers-strategy=NONE`) and directly use `reactor.netty.http.server.HttpServer` forwarded header support to change the HTTP configuration.
+
+
+
 [[howto.webserver.enable-multiple-connectors]]
 == Enable Multiple Connectors