It's really easy to update the status message of a request. In this example, I want to add a link to our help doc:
Go to the request, find the "Actions" panel in the top right corner and select the status the request is already set to. In this case, since I want to edit the status of a Released request, I'll re-select "Released":
Make your edits in the "Customize response" text area, and uncheck "Email your response...":
Update the request status, and your new status message will be saved without sending any notifications.