Issue title: Payload clarification on sendNotification
 Issue body: Hi - can you clarify the payload for sendNotification? In the spec is it shown as a string yet in the comment you refer to specifying a JSON payload? At the moment, if I try and send a JSON payload as a string into that parameter then the output received is as shown below. This means that the Logic app which receives it cannot parse the JSON value of the payload parameter. FYI I am sending this in a UdF by defining a JSON object and then calling JSON.stringify on it and setting that as the value of the payload parameter.

{
  "TopologyObjectId": "50be4ee1-244a-4b98-9084-03c13148aec7",
  "ResourceType": "Space",
  "Payload": "{\"space\":\"Home office\",\"status\":\"Booked\"}",
  "CorrelationId": "55e8bd55-7b8f-4796-ade3-3acd336ccb9c"
}

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 411be9a3-a430-4a49-9730-11b4e9d5b512
* Version Independent ID: c9c58d4b-e391-c128-5616-421a69228b4d
* Content: [How to use user-defined functions in Azure Digital Twins](https://docs.microsoft.com/en-us/azure/digital-twins/how-to-user-defined-functions#Client-Reference)
* Content Source: [articles/digital-twins/how-to-user-defined-functions.md](https://github.com/Microsoft/azure-docs/blob/master/articles/digital-twins/how-to-user-defined-functions.md)
* Service: **digital-twins**
* GitHub Login: @alinamstanciu
* Microsoft Alias: **alinast**
 Comments: 
 Comment 0: @davebanthorpe Thanks for the feedback! We are currently investigating and will update you shortly.
 Comment 1: This is a test comment please disregard
![alt text](https://wtv3.blob.core.windows.net/media/bingspellcheck.png?sv=2017-04-17&sr=b&sig=142UFC9mnSHQFcXKM82inzSK8VNXT6z04wl9wUlqSSc%3D&st=2018-12-04T20%3A41%3A32Z&se=2021-12-04T20%3A46%3A32Z&sp=rw "BingSpellCheck.PNG")[Test Url](https://github.com/MicrosoftDocs/azure-docs/issues/19946)
 Comment 2: @davebanthorpe Thanks for the feedback! I have assigned the issue to the content author to investigate further and update the document as appropriate

@alinamstanciu Hi, can you please check this issue and update the document as necessary? Thanks You!
 Comment 3: Hi @davebanthorpe! Happy New Year! Thanks for your questions and interest! I regret the longer than usual response time since most of the team has been out. I just recently rejoined the Digital Twins team and updated the [How to UDF](https://docs.microsoft.com/azure/digital-twins/how-to-user-defined-functions) documentation. You'll see that in the [How to UDF](https://docs.microsoft.com/azure/digital-twins/how-to-user-defined-functions
), the [Tutorial example](https://github.com/Azure-Samples/digital-twins-samples-csharp/blob/master/occupancy-quickstart/src/actions/userDefinedFunctions/availabilityForTutorial.js), and the new [client library reference](https://docs.microsoft.com/azure/digital-twins/reference-user-defined-functions-client-library) the Payload is to be converted into String format. Please make sure that you follow the [Tutorial](https://docs.microsoft.com/azure/digital-twins/tutorial-facilities-events) - specifically **8.c** which describes how to correctly configure the Logic App to parse the JSON and to send the output to your email.
 Comment 4: @davebanthorpe - Please let me know if you have any other questions regarding the examples, documentation, or reference material! I'm happy to further clarify as needed. Also, a full refresh and update of the four tutorial documents has been completed and will be brought live soon.
 Comment 5: We will now proceed to close this thread. If there are further questions regarding this matter, please tag me in your reply. We will gladly continue the discussion and we will reopen the issue.
 Comment 6: Sorry, for late response, I didn't see this until few days back. You can have any payload in the SendNotification. For example based on our article, you can see we have a json

    var alert = {
      message: 'Temperature reading has surpassed threshold',
      sensorId: telemetry.SensorId,
      reading: parseReading
    };

sendNotification(telemetry.SensorId, "Sensor", JSON.stringify(alert));

See article https://github.com/MicrosoftDocs/azure-docs/blob/master/articles/digital-twins/reference-user-defined-functions-client-library.md#sendnotificationtopologyobjectid-topologyobjecttype-payload
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