Move Activity comment to Request body payload

[``* `#13954` Add, edit, and quote comments in work package overview``](https://www.openproject.org/work_packages/13954)

The `Parameters` keyword defines URI Parameters:
https://github.com/apiaryio/api-blueprint/blob/master/API%20Blueprint%20Specification.md#def-uriparameters-section
(these should match the URI Template)

POST data should be specified in the Request section.