[DevKit] docs - issue receipt - the documentation does not talk about the options for the "Accept" header AND the InStore app does use an appId as query parameter that is not documented

[turtletramp]

What the docs provide:

What we do in the InStore App (see the AcceptHeader + the appId at the end of the query string):

    public async Task<byte[]> GetPNGReceiptAsync(IssueResponse issueResponse)
    {
        var httpRequest = new HttpRequestMessage(HttpMethod.Get, $"/v2/issue/{issueResponse.ftQueueID}/{issueResponse.ftQueueItemID}?appId={Guid.NewGuid()}");
        httpRequest.Headers.Add("Accept", "application/png");
        var result = await SendWithAuthenticatedClient(httpRequest);
        if (!result.IsSuccessStatusCode)
        {
            throw new Exception("Failed to receive PNG call." + await result.Content.ReadAsStringAsync());
        }
        return await result.Content.ReadAsByteArrayAsync();
    }

TODO:

• extend the docs to inform about the different supported Accept headers to define the wanted returned format
• either document the appId query parameter OR remove it from the InStore App in case it makes no sense (as it anyway does create a new GUID with every call)