Improve docs on forwarded headers

Issue: SPR-16660

src/docs/asciidoc/web/webflux.adoc

@@ -854,6 +854,44 @@ views through the <<webflux-config-view-resolvers,WebFlux Config>>. Default view
 always selected and used if they match the requested media type.
 
 
+[[webflux-filters]]
+== Filters
+[.small]#<<web.adoc#filters,Same in Spring MVC>>#
+
+As part of the <<webflux-web-handler-api>>, the `spring-web` module provides a number of
+`WebFilter` implementations.
+
+
+
+[[webflux-filters-forwarded-headers]]
+=== Forwarded Headers
+[.small]#<<web.adoc#filters-forwarded-headers,Same in Spring MVC>>#
+
+As a request goes through proxies such as load balancers the host, port, and
+scheme may change presenting a challenge for applications that need to create links
+to resources since the links should reflect the host, port, and scheme of the
+original request as seen from a client perspective.
+
+https://tools.ietf.org/html/rfc7239[RFC 7239] defines the "Forwarded" HTTP header
+for proxies to use to provide information about the original request. There are also
+other non-standard headers in use such as "X-Forwarded-Host", "X-Forwarded-Port",
+and "X-Forwarded-Proto".
+
+`ForwardedHeaderFilter` detects, extracts, and uses information from the "Forwarded"
+header, or from "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto".
+It wraps the request in order to overlay its host, port, and scheme and also "hides"
+the forwarded headers for subsequent processing.
+
+Note that there are security considerations when using forwarded headers as explained
+in Section 8 of RFC 7239. At the application level it is difficult to determine whether
+forwarded headers can be trusted or not. This is why the network upstream should be
+configured correctly to filter out untrusted forwarded headers from the outside.
+
+Applications that don't have a proxy and don't need to use forwarded headers can
+configure the `ForwardedHeaderFilter` to remove and ignore such headers.
+
+
+
 
 
 [[webflux-controller]]

src/docs/asciidoc/web/webmvc.adoc

@@ -1029,6 +1029,7 @@ Once the Servlet 3.0 configuration is in place, simply add a bean of type
 
 [[filters]]
 == Filters
+[.small]#<<web-reactive.adoc#webflux-filters,Same in Spring WebFlux>>#
 
 The `spring-web` module provides some useful filters.
 
@@ -1048,8 +1049,9 @@ available through the `ServletRequest.getParameter{asterisk}()` family of method
 
 
 
-[[filters-forwarded-headers]]
+[[webflux-filters-forwarded-headers]]
 === Forwarded Headers
+[.small]#<<web-reactive.adoc#webflux-filters-forwarded-headers,Same in Spring WebFlux>>#
 
 As a request goes through proxies such as load balancers the host, port, and
 scheme may change presenting a challenge for applications that need to create links