minor symfony#17982 [HttpFoundation] Add missing RFC comment (dosten)

This PR was merged into the 2.3 branch.

Discussion
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[HttpFoundation] Add missing RFC comment

| Q             | A
| ------------- | ---
| Branch        | 2.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

See https://tools.ietf.org/html/rfc7725

Commits
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7901307 Add missing RFC comment

Author: fabpot

src/Symfony/Component/HttpFoundation/Response.php

@@ -109,7 +109,7 @@ class Response
         428 => 'Precondition Required',                                       // RFC6585
         429 => 'Too Many Requests',                                           // RFC6585
         431 => 'Request Header Fields Too Large',                             // RFC6585
-        451 => 'Unavailable For Legal Reasons',
+        451 => 'Unavailable For Legal Reasons',                               // RFC7725
         500 => 'Internal Server Error',
         501 => 'Not Implemented',
         502 => 'Bad Gateway',