AI can often synthesis information out of for example, code and screenshots and navigate a website. It could effectively document the current state of a given web application for example whereas most companies have 0 documentation whatsoever.
I'd take AI generated slop reviewed by the person who created the system over tech writer babble any day of the week.
I'm sure I'm not the only one who was reading about some interesting but flawed system only to discover later that they were talking about MY OWN SOFTWARE!? (only half-joking here)
AI-made documentation has 0% of the quality.
As the OP pointed, AI can only document things that somebody already wrote down. That's no documentation at all.