Document fields of External API
The External API specification does not specify the exact fields of responses or formats expected in form parameters.
This leaves room for interpretation which is undesirable.
The goal of this task is to extend the existing specification by including listing of the fields. For this purpose, I will use the current state of develop as reference and basically document the current behaviour (which at least in theory should be 1.0.0.)
The pull request has been merged. I therefore resolve this issue as "Fixed and reviewed"