minor #17525 Fixed the phpDoc of UserInterface (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

Fixed the phpDoc of UserInterface

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #13037
| License       | MIT
| Doc PR        | -

#13037 is the original issue ... which was tried to be fixed in #13146 ... which was closed without merging it.

Commits
-------

8e2a33e Fixed the phpDoc of UserInterface

src/Symfony/Component/Security/Core/User/UserInterface.php

@@ -47,7 +47,7 @@ interface UserInterface
      * and populated in any number of different ways when the user object
      * is created.
      *
-     * @return Role[] The user roles
+     * @return (Role|string)[] The user roles
      */
     public function getRoles();