Add documentation for REST API response codes

I'm using parentheses to differentiate between those responses that have an actual status message and those that don't, In this case the code is just doing a `res.send(200, notificationStream);` so there's no explicit message. In other cases the code sends a specific status message `return res.send(err.code, err.msg);` and this those cases I've included the actual messages without parentheses. There are a few other cases where we don't have a specific status message (mostly `201` status codes) and those have parentheses as well.