[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        | 
Yuck, that's ugly. 

Even if PSR-5 is not an accepted standard yet, some tools are implementing it already: https://github.com/phpDocumentor/phpDocumentor2/issues/607 (most likely because the author is involved in PSR-5 ;) ).