minor #21495 Add HEADER_FORWARDED to setTrustedHeaderName docs (pascal-hofmann)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #21495).

Discussion
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Add HEADER_FORWARDED to setTrustedHeaderName docs

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

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Commits
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b46276e Add HEADER_FORWARDED to setTrustedHeaderName docs

src/Symfony/Component/HttpFoundation/Request.php

@@ -596,6 +596,7 @@ class Request
      *  * Request::HEADER_CLIENT_HOST:  defaults to X-Forwarded-Host  (see getHost())
      *  * Request::HEADER_CLIENT_PORT:  defaults to X-Forwarded-Port  (see getPort())
      *  * Request::HEADER_CLIENT_PROTO: defaults to X-Forwarded-Proto (see getScheme() and isSecure())
+     *  * Request::HEADER_FORWARDED:    defaults to Forwarded         (see RFC 7239)
      *
      * Setting an empty value allows to disable the trusted header for the given key.
      *