minor #40728 [PHPDoc] Fix some union type cases (fancyweb)

This PR was merged into the 4.4 branch.

Discussion
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[PHPDoc] Fix some union type cases

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

While working on symfony/symfony#40154, I discovered some PHPDoc issues, I'm going to comment in the review. Upper branches will need some fixes too.

Commits
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dd1481642b [PHPDoc] Fix some union type cases

Response.php

@@ -123,7 +123,7 @@ public function getHeaders()
      * @param string $header The header name
      * @param bool   $first  Whether to return the first value or all header values
      *
-     * @return string|array The first header value if $first is true, an array of values otherwise
+     * @return string|array|null The first header value if $first is true, an array of values otherwise
      */
     public function getHeader($header, $first = true)
     {