API v2: Change boolean value to use true/false

While this is not the OpenProject API, we use true/false for other APIs and for other parameters in the application, like `layout=false`. true/false also seems more intuitive.
API v2: Document some useless code
Merge pull request #1911 from opf/hotfix/api_v2_rewire_parents_value

API v2: Use true/false as rewire parents value