Update IoT Hub Rest API documentation

The rest api documentation is out of date: https://docs.microsoft.com/en-us/rest/api/iothub/?source=docs (last updated on 09/30/2015)

We are looking for IoT Hub failover operation described in
https://github.com/Azure/azure-rest-api-specs/blob/master/specification/iothub/resource-manager/Microsoft.Devices/stable/2020-08-01/iothub.json

"/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Devices/IotHubs/{iotHubName}/failover"

Compare the published IoT Hub REST API documentation at the linked Microsoft Docs URL with the failover operation in specification/iothub/resource-manager/Microsoft.Devices/stable/2020-08-01/iothub.json. Start with the /failover entry; done means the REST documentation accurately describes that IoT Hub failover operation.

Written by the indexing model from the issue text.