I'd break things down into two broad kind of documentation:
1. Comments on code
2. User-facing manuals
There's no need for an LLM to write the former. Comments are left by a person to explain decisions when they're no longer present. LLMs don't go away. You can always ask the LLM why the code was written that way.
As for users... they famously don't read. UX designers take that into account, and try to make their interfaces self-documenting. A user who can't figure it out from the design in front of them probably won't find the right part of the manual to read, either. They'd rather dive in than read cover-to-cover.
The "for dummies" books seem to sell well. I wonder if you could get an LLM to write those.