minor #13211 [Security] fixed wrong phpdoc (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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[Security] fixed wrong phpdoc

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12597
| License       | MIT
| Doc PR        | n/a

Commits
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064062d [Security] fixed wrong phpdoc

src/Symfony/Component/Security/Core/Authentication/Token/TokenInterface.php

@@ -47,8 +47,10 @@ interface TokenInterface extends \Serializable
     /**
      * Returns a user representation.
      *
-     * @return mixed either returns an object which implements __toString(), or
-     *               a primitive string is returned.
+     * @return mixed Can be a UserInterface instance, an object implementing a __toString method,
+     *               or the username as a regular string
+     *
+     * @see AbstractToken::setUser()
      */
     public function getUser();