minor #12168 [HttpFoundation] CSRF warning docs on Request::enableHttpMethodParameterOverride() (Kristof Van Cauwenbergh)

This PR was merged into the 2.3 branch.

Discussion
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[HttpFoundation] CSRF warning docs on Request::enableHttpMethodParameterOverride()

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12043
| License       | MIT
| Doc PR        | /

Since I wanted to understand this issue I did some research and altered the comment block. Is this a clear enough explanation or does it need more?

Commits
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deb70ab CSRF warning docs on Request::enableHttpMethodParameterOverride()

src/Symfony/Component/HttpFoundation/Request.php

@@ -648,6 +648,9 @@ class Request
      *
      * Be warned that enabling this feature might lead to CSRF issues in your code.
      * Check that you are using CSRF tokens when required.
+     * If the HTTP method parameter override is enabled, an html-form with method "POST" can be altered
+     * and used to send a "PUT" or "DELETE" request via the _method request parameter.
+     * If these methods are not protected against CSRF, this presents a possible vulnerability.
      *
      * The HTTP method can only be overridden when the real HTTP method is POST.
      */