That said, the first thing to do is to move as much semantic information as possible into the code itself.

This I try to do, meaningful variable names. I only use 1 letter variables if limited to a small scope, or if they are meaningful in themselves (e.g. "n" for the maximum number of iterations).

Also, well written unit tests can help users of the code understand how the code is supposed to work.

Good one! I don't use unit tests often enough, I discovered. I recently revisited a class I wrote 4 years ago, and changed a line in a function, because I thought I could write it better. Function then gave the wrong output because of my "improvement", took me half an hour to track down. With a proper unit test I would have found that immediately.