Add note comment for the in-memory user-service.js
Add note comment for the in-memory user-service.js

GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services,GoogleCloudPlatform/professional-services