--output option with --append and either --fmt json or change in format is unsupported

[tjayrush]

And I am explicitly declaring this to be okay. We won't disallow it in code. We won't try to protect the end user from getting themselves into trouble. We won't write code that reads in the existing json and then re-export it with additional data. We will simply allow the user to create invalid data. We will document this in the documentation for --append.