[APIv3] Specify first iteration of WorkPackage PATCH-Endpoint

This PR belongs to [#16770](https://community.openproject.org/work_packages/16770).

It defines the very first portion of the `PATCH` endpoint for work packages.
This is an example and I hope nobody reads the message text in an example as part of the specification (as in: If the message is different it is a violation).
I'd rather keep this message as an example, because I'd really wish this *was* the actual error message.

Also: I'd go nuts if I had to change every error message when the implementation for that changes.