Ask HN: Best documentation in code you have seen?

Qt has amazing API documentation.

Hands down: JDK.

This is interesting - when I read your post, I thought, "What a strange question. The best documentation is just the one that explains the problem in a simple, easy to understand manner." You see, I was thinking "in code" as in "// This comment explains the logic in the next code block". And then when I read the comments from others, at the time of this response everyone has chosen that the word "documentation" refers to a "body of knowledge that ships with the product". It's funny how we all interpret this differently.