Document the error that comes from the order of the worker runs in the json fixture file

This happens pretty often when we regenerate the fixtures, and there isn't much we can do about it, but it would be nice to document it somewhere so people are not confused and know how to fix it (by moving the worker runs around in the json so they are in the right order)

rest_framework.exceptions.ValidationError: [ErrorDetail(string="Can't add or update WorkerRun 0239ce26-d3f8-44ed-b38d-f6c792e049e6 because parents field isn't properly defined. It can be either because one or several UUIDs don't refer to existing WorkerRuns or either because listed WorkerRuns doesn't belong to the same Process than this WorkerRun.", code='invalid')]