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If the documentation and code could be in-sync, then the documentation would just be code, like type hints. But good documentation that the parent is talking about cannot be in-sync.

Programming languages can't understand semantics, and that's why we program in the first place. I can't tell a computer "I would like a program to achieve this goal", instead I have to instruct it how to achieve the goal. Then, I would need to document elsewhere what the goal is and why I'm doing it.

LLMs change that, we can now legitimately ask the model "I would like a program for this goal". But the documentation is lost in the code if we don't save comments or save the prompt.

Git commits are also a good source of documentation. They shouldn't describe what we're doing, because I can just read the code. But often, I come across code and I'm thinking "why are we doing this? Can I change this? If I change it, what are the side effects?" If I'm lucky, the git blame will answer those questions for me.