AI is most excellent at reading and understanding large codebases and, with some guidance docs, can easily reproduce accurate technical documentation. Divide and conquer.
Accuracy/faithfulness to the code as written isn't necessarily what you care about though, it's an understanding of the underlying problem. Just translating code doesn't actually help you do that.
Again, nobody would spend a dime to create the technical documentation, even if it could be done somewhat faster with AI support. Also, in my experience AI is not so great explaining the consequences to business processes when documenting code.