Differentiate Forwarded and X-Forwarded headers in proxy docs

[skdas20]

See gh-19461.

The proxy server section currently says:

To fix this, you can use RFC 7239 to specify that a load balancer is being used.
To make the application aware of this, you need to configure your application server to be aware of the X-Forwarded headers.

which points at RFC 7239 and then, in the next sentence, refers to the X-Forwarded-* headers as though they were the same mechanism. They are not: RFC 7239 defines a single standard Forwarded header, while X-Forwarded-Host / X-Forwarded-Proto / X-Forwarded-For are the older non-standard set.

This change:

• describes the two kinds of headers separately, so a reader can tell which applies to their deployment;
• notes that most proxies still send X-Forwarded-* while Spring Framework and servers such as Reactor Netty and Jetty understand both, so neither can be assumed to be the only one in use;
• adds a note that both kinds are client-supplied unless a proxy overwrites them, and states that the edge proxy has to drop or overwrite untrusted values for both kinds — dropping only one set still leaves the application spoofable through the other;
• mentions that ForwardedHeaderFilter / ForwardedHeaderTransformer handle both kinds and can be configured to remove the headers instead of applying them.

The wording follows the framing in the issue. Only features/exploits/http.adoc needed changing — the servlet and reactive pages xref this section rather than repeating it.

Happy to adjust the emphasis or trim it if you had a different structure in mind; this is meant as a starting point rather than a finished opinion on the guidance.