Changes to Flush 204 return code description (Azure#10217)

Author: lajillel

specification/storagecache/resource-manager/Microsoft.StorageCache/stable/2020-03-01/storagecache.json

@@ -586,7 +586,7 @@
             "schema": {}
           },
           "204": {
-            "description": "Cache flushed.",
+            "description": "Cache flush already in progress or has completed.",
             "schema": {}
           },
           "default": {