[Security] Missing return type in "UserInterface"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/13037
| License       | MIT
| Doc PR        | 
First of all it's not ugly and it's well supported for years by IDE (which opens my eyes on fact how stupid it is to create new format...). Second, that `(|)[]` PSR format doesn't fit at all as it's described as "specified as containing multiple types" because that specific return cannot/shouldn't contain mixed strings and objects in one array as it would create a mess. My change represents and documents 2 completely different outputs. That's my IMHO guys.