Role Assignment PUT only provides 201 response

In the Role Assignment Create operation: https://github.com/Azure/azure-rest-api-specs/blob/aab4327f48563f5733bad5a9e403b5c6a1dd2e37/specification/authorization/resource-manager/Microsoft.Authorization/stable/2015-07-01/authorization.json#L404-L452 Only the 201 response status is described. In general, PUT is supposed to be an idempotent operation. I would expect that if the resource that is being PUT already exists, the server would respond with a 200 or a 204, yet neither of those response codes are not detailed.

Related to Azure/azure-cli#8568

Open specification/authorization/resource-manager/Microsoft.Authorization/stable/2015-07-01/authorization.json around lines 404-452 and inspect the Role Assignment Create PUT operation. Compare nearby PUT operations and the related Azure CLI issue to determine the documented success responses; done means the specification describes the applicable existing-resource responses and passes repository validation.

Written by the indexing model from the issue text.