Flesh out documention on how `json-body` works

Currently we provide a few examples of using `--json-body` with CLI commands in https://docs.oxide.computer/cli/guides/introduction, but don't get into the details of how we expect it to be used.

We should document:

* When a value is specified as an `--<ARG>` and in the JSON body, which one is used?
* Which commands require it?
* Guidance on when we recommend using it.