updated public doc for export put etag optional (#20218)

Author: bvishali

specification/cost-management/resource-manager/Microsoft.CostManagement/stable/2021-10-01/costmanagement.exports.json

@@ -162,7 +162,7 @@
           "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/"
         },