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)
``params`` contains the parsed body/query parameters. But here we want Representable to do the job. Thus, we use [the API data provided by Grape](https://github.com/intridea/grape#api-data-formats).