Oh gosh, yes. I've found that approach to be the most helpful. In a year (or three?), I know that I won't remember anything about what it was supposed to do. I'm pretty sure I'm stupid or forgetful. Probably both.

I like your assertion of documentation being a measure of progress. As dumb as it sounds, even if you're the only consumer of an API you wrote internally for some on-off application, documenting it thoroughly is a tremendous time saver. Granted, I still slip up occasionally, especially if it's a quick script of sorts, but I find more often than not that when I do regret not having taken the time to document something, it's (now) usually something deceptively simple that I wish I had better curated, because it eventually finds new life as part of a greater work.