This helps to avoid the following deprecation message being generated by Symfony's DebugClassLoader:

> Method "JsonSerializable::jsonSerialize()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Solarium\Core\Query\AbstractDocument" now to avoid errors or add an explicit @return annotation to suppress this message.

Co-authored-by: Markus Kalkbrenner <git@kalki.de>