Improve OpenAPI examples of ToMany relations

So far, we have specified the examples for ToMany relations like `example: '["/camp_collaborations/1a2b3c4d"]'`.
If we instead specify them as follows: `example: ['/camp_collaborations/1a2b3c4d']`, the swagger docs give a better description of reality. Compare the example for campCollaborations and for categories:

Also, [Camp#campCollaborations](https://github.com/ecamp/ecamp3/blob/devel/api/src/Entity/Camp.php#L60..L64) does not have any example and description (multiline comment) yet, we should add that.