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)
It took me quite some time to figure out that this is used to translate between the method name in the form and the name in the AR model. This is made worse by this method defining the structure and ```readonly_attributes_unchaged``` magically knowing about it.

Either (could be more than one) we:
* check whether reform provides the functionality of getting the two corresponding method names
* move the translation into it's own class which allows to query for the other name
* add a comment

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I was surprised to see done_ratio and estimated_time in here. Is that just to check whether the implementation works for more attributes? Because strictly speaking this is out of the scope of the PR.