[phpDoc]: fixed reference to Product (should be ProductInterface) in Pro...

...ductManager and ProductRepositoryInterface

| Q                    | A
| -------------------- | ---
| Bug fix?             | no
| New feature?         | no
| BC breaks?           | no
| CI currently passes? | should
| Tests pass?          | should
| Scenarios pass?      | should
| Checkstyle issues?*  |
| PMD issues?**        | 
| Changelog updated?   | needed ?
| Fixed tickets        | n/a
| DB schema updated?   | no
| Migration script?    | not needed
| Doc PR               | yes