Incorrect Documentation, Cannot Update existing Cloud Service

Documentation is incorrect for New-AzCloudService. It cannot be used to update a Cloud service (though that is the premise of the first sentence). The script will throw an error if the Cloud Service exists.

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#### Document details

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* ID: 7ac3e71b-7d9e-c221-08d2-fe4770905281
* Version Independent ID: fc9ecdfd-2ead-37c5-b12c-eaa2224edb74
* Content: [New-AzCloudService (Az.CloudService)](https://docs.microsoft.com/en-gb/powershell/module/az.cloudservice/new-azcloudservice?view=azps-6.4.0)
* Content Source: [src/CloudService/help/New-AzCloudService.md](https://github.com/Azure/azure-powershell/blob/main/src/CloudService/help/New-AzCloudService.md)
* Service: **azure-powershell**
* GitHub Login: @dcaro
* Microsoft Alias: **dcaro**

Start with src/CloudService/help/New-AzCloudService.md and compare its opening description with the reported behavior when the cloud service already exists. Update the documentation so it accurately describes whether existing services can be updated and the error scenario, then verify the rendered New-AzCloudService page reflects the corrected guidance.

Written by the indexing model from the issue text.