I agree, typing up a polished design doc should be enough. That's not something you can throw together on your last day though. You get a polished design doc by

I agree, typing up a polished design doc should be enough. That's not something you can throw together on your last day though. You get a polished design doc by maintaining the doc and your software simultaneously. No, it's not like you're working full time. Anyone can offer to help out a couple hours a week after they leave. It's not just good service, but it's also the most intelligent thing we can do: the original writer of software is by far in the best position to maintain it. If we all did this, we'd all benefit.