In those constrained circumstances, what I want most is a high level explainer of what the system is supposed to do, how it does it, and any relevant background

In those constrained circumstances, what I want most is a high level explainer of what the system is supposed to do, how it does it, and any relevant background information on why it does its thing in this particular way. "this service sits between x and y, doing z. Z is handled by libraries a and b. We couldn't use c because reason d. This is how you call it, and this is an example result. You'll notice that we're passing both the user id and email address in the request, because we didn't want this service to talk to the database. Oh also this whole thing depends on a local sqlite cache because shrug"