REST GET .../submissions should return instanceName or update API docs

[sadiqkhoja]

Currently REST endpoint GET forms/id/submissions doesn't return instanceName, we need to fix that or update API docs where we say that instanceName is only supported for ODATA endpoints

curl 'https://test.getodk.cloud/v1/projects/390/forms/Simple/Submissions' -H  ...

Response:
[
    {
        "instanceId": "uuid:08349870-0941-4a7c-8843-11fdd9a70908",
        "submitterId": 17150,
        "deviceId": null,
        "createdAt": "2022-08-30T15:34:22.162Z",
        "updatedAt": null,
        "reviewState": null
    }
]

REF: https://forum.getodk.org/t/odk-central-api-not-returning-instancename-of-submission/38729/6

[matthew-white]

instanceName is also available using the REST API endpoints for a submission's versions or for a particular submission version. I think this relates to #481: the API distinguishes between submissions and submission versions (the two return different data), but the API docs don't always reflect that.