16875 adding deleting parent and child relations not working in details pane

[``* `#16875` Adding/Removing parent relations not working in details pane``](https://community.openproject.org/work_packages/16875)
[``* `#16878` Removing child relations not working in details pane``](https://community.openproject.org/work_packages/16878)
The documentation would then not match the actual implementation:
* child and parent parameter not documented for work package PATCH
* various attributes declared as writable which are not for work package PATCH
@NobodysNightmare you got anything that could serve as doc for this?