Personally, I don't think that LLMs are useful for documentation. I feel that it only helps you have documentation and won't help with actual useful documentation. Just a single hallucinated aspect of a documentation renders it almost useless.

In my experience it only generates adequate documentation when the software is simple and already self explanatory. It does not however help with actual use cases that might need to be documented because they are somehow difficult.

Software developers that are not interested in providing documentation will probably also not be interested in checking whether the documentation slopped out by an LLM is actually correct.