Fix misleading TokenInterface docblock

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |

I've found the docblock for `TokenInterface::getUser` misleading. By reading it I've written something like:
```php
$username = (string) $token->getUser();
```
This actually doesn't work, since `UserInterface` doesn't implements `__toString` and I have some instance of `AdvancedUser` (coming from InMemory provider).

If the token always return a `UserInterface` or string, then my fix is ok.