The most reliable non-fine-tuned method I have seen is to do many, many passes over the doc, instructing the LLM to focus on only one rule during each pass.
I recently produced a bunch of migration guides for our project by pointing Claude 4 Sonnet at my poorly structured Obsidian notes (some more than 5 years old), a few commits where I migrated the reference implementation, and a reasonably well-maintained yet not immediately actionable CHANGELOG. I think the result is far from top-notch but, at the same time, it is way better IMO than nothing (nothing being the only viable alternative given my priorities): https://oslc.github.io/developing-oslc-applications/eclipse_...
Looking forward to your model/product!
P.S. https://www.gov.uk/guidance/style-guide/technical-content-a-... also looks useful
And here are its style comments: https://gist.github.com/stevelandeydescript/a586e312c400769b...
I don't plan to release the code, since I don't really want my docs to be written in this voice. But it doesn't feel entirely unhelpful, as long as I'm personally curating the changes.