minor [Serializer] Fix CS. Uniformize PHPDoc.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
this change is actually wrong (PHP-CS-Fixer will revert it the next time we run it though)