>Suppose a company wants to write some product documentation so customers and prospective customers know what features their product has and how to use them. They hire someone to write the documentation and then someone else to distribute it. Is there a good way to distinguish this from advertising?
Yes. You self host it as a company, and it can only be reproduced (if they wish) in outlets (say review sites) when there's no payment or compensation of any kind involved for that.
It's a corporation though. It can't do anything without paying someone to do it, unless someone volunteers to do it for free, which isn't very likely. And how do you self-host distribution? You would have to run your own fiber to every customer's house or spin up your own postal service or you're paying someone to do that.