Update ProximityPlacementGroupType description to scope out 'Ultra' from usage

[aspand]

Latest improvements:

This PR for updating description of ProximityPlacementGroupType (added in #5507). ProximityPlacementGroupType can assume value 'Standard' or 'Ultra'. ProximityPlacementGroupType 'Ultra' is not supported. Support for 'Ultra' shall be added in future .
The PR is to add this detail in description of ProximityPlacementGroupType so that user can be informed (from documentation generated using the swagger spec) to not use 'Ultra'.

Contribution checklist:

• I have reviewed the documentation for the workflow.
• Validation tools were run on swagger spec(s) and have all been fixed in this PR.
• The OpenAPI Hub was used for checking validation status and next steps.

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• adding/removing properties.
• adding/removing API-version.
• adding a new service in Azure.

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[openapi-sdkautomation]

SDK Automation [Logs] (Generated from 451b51f)

Python: Azure/azure-sdk-for-python

• Package generation pending.

Java: Azure/azure-sdk-for-java

• Package generation pending.

JavaScript: Azure/azure-sdk-for-js

• Package generation pending.

Go: Azure/azure-sdk-for-go

• Package generation pending.

Ruby: Azure/azure-sdk-for-ruby

• Package generation pending.

[AutorestCI]

Automation for azure-sdk-for-ruby

Nothing to generate for azure-sdk-for-ruby

[AutorestCI]

Automation for azure-sdk-for-python

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-python#5853

[AutorestCI]

Automation for azure-sdk-for-go

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-go#5108

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[azuresdkci]

Can one of the admins verify this patch?

[kpajdzik]

Do you want to fix Model Validation errors in this PR?

[aspand]

Do you want to fix Model Validation errors in this PR?

The scope of the PR is to just add the description

[steveesp]

Only the description should be changed, as follows:
Ultra – For future use