I tend to document some tricky non-obvious pieces of knowledge directly above the relevant code. "We have to do X below instead of obvious-first-idea-Y because Z".

Any time a refactoring comes up which moves code around, AI (or my coworkers) remove those comments without thinking twice, and I need to tell them "hey this is still valid".

It's kind of a learning JIT. It's no use to go through and memorize something you don't need in the short term. It's hard to memorize well and by the time you need to draw on the knowledge it's already hazy. This is why you can think of such documentation more as a reference manual and not just plain documentation.

In any case, AI is great for traversing a codebase and producing at least a draft of such documentation.