minor symfony#25455 [HttpFoundation] Add immutable to setCache's PHPDoc (dunglas)

This PR was merged into the 3.4 branch.

Discussion
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[HttpFoundation] Add immutable to setCache's PHPDoc

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
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6e248ca [HttpFoundation] Add immutable to setCache's PHPDoc

Author: nicolas-grekas

src/Symfony/Component/HttpFoundation/Response.php

@@ -952,7 +952,7 @@ public function setEtag($etag = null, $weak = false)
     /**
      * Sets the response's cache headers (validation and/or expiration).
      *
-     * Available options are: etag, last_modified, max_age, s_maxage, private, and public.
+     * Available options are: etag, last_modified, max_age, s_maxage, private, public and immutable.
      *
      * @param array $options An array of cache options
      *