"Hi Star Developer,
just a little informal heads-up to tell you that from next week, we'll be requiring documentation of code, and helpful commenting in the code - it's going to be company policy, and there'll be no exceptions"
From then on, you just deal with that failure the same as you would deal with a failure to turn up on time, a failure to stop goofing off during work, etc. Bottom line is if the boss says document, you document or you're not doing your job properly.
