16983 api extend wp patch for description

[``* `#16983` Extend API V3's work package PATCH endpoint for description``](https://community.openproject.org/work_packages/16983)
I like the direction this is heading but I think that as of now, not all validations, that could prevent a work package from being saved are covered by the model yet. So errors, raised when saving, will probably not be shown in the API then. Or reform contracts also include the errors from their wrapped AM, but I don't think so. So we either merge the errors here or provide a way to get the merged errors.