Have you never had a situation where a question arose a year (or several) later that wasn’t addressed in the original documentation?
In particular IME the LLM generates a lot of documentation that explains what and not a lot of the why (or at least if it does it’s not reflecting underlying business decisions that prompted the change).
You can ask it to generate the why, even if it the agent isn’t doing that by default. At least you can ask it to encode how it is mapping your request to code, and to make sure that the original request is documented, so you can record why it did something at least, even if it can’t have insight into why you made the request in the first place. The same applies to successive changes.