message: updated public doc for export put etag optional
diff: "Exports"
  ],
  "operationId": "Exports_CreateOrUpdate",
  - "description": "The operation to create or update a export. Update operation requires latest eTag to be set in the request. You may obtain the latest eTag by performing a get operation. Create operation does not require eTag.",
  + "description": "The operation to create or update an export. You can optionally provide an eTag if desired as a form of concurrency control. To obtain the latest eTag for a given export, perform a get operation prior to your put operation.",
  "externalDocs": {
  "url": "https://docs.microsoft.com/en-us/rest/api/costmanagement/"
  },
  