True story: I took over a 10,000 node network for an admin that had just killed himself, leaving no server or network documentation whatsoever. It took time to reflash all of the switches and find tricks to replace passwords on servers and figure out how everything was more or less organized. Leaving a few passwords for the next guy and a rough, top-down analysis of how things interact on paper will work wonders for the next guy. That being said, it's possible (not pleasant) to move forward with no prior knowledge; any explanation you can provide will go a long way. Doubly-so if you put it on paper.
You have to realize that gaps will be filled with his way of doing things, not yours. In a few years the network as you knew it could be almost unrecognizable, but it's for the best, and the new guy will have complete control of what's there and how it interacts.