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)
@hschink Actually it was more straight-forward than I thought to correct this (reading the ActionView docs helped :smile:) See de1ebfbe4cb3622a70d12410a5c532a8f08d7ceb