AKS REST spec: please mention redonly properties

[aelmanaa]

Hello

as discussed in this issue https://github.com/MicrosoftDocs/azure-docs/issues/47422#issuecomment-583561810 , it would be nice to mention which properties cannot be updated once the cluster is created.
For example in https://github.com/Azure/azure-rest-api-specs/blob/master/specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2020-01-01/managedClusters.json#L1018 , CIDR properties should be mentionned as readOnly

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @Azure/aks-pm.