updated login help text (Azure#15609)

Author: DaeunYim

specification/mysql/resource-manager/Microsoft.DBforMySQL/stable/2017-12-01/mysql.json

@@ -1987,7 +1987,7 @@
       "properties": {
         "administratorLogin": {
           "type": "string",
-          "description": "The administrator's login name of a server. Can only be specified when the server is being created (and is required for creation)."
+          "description": "The administrator's login name of a server. Can only be specified when the server is being created (and is required for creation). The login name is required when updating password."
         },
         "administratorLoginPassword": {
           "type": "string",