> My take is that pay vs free software and good vs bad documentation are orthogonal and you can be in any of the four quadrants.

It's definitely not independent. Few people enjoy writing documentation and even then, it takes a lot of time. Plus, writing good documentation is something you need to train for. Very few OS people want to spend their free time writing code and then spend just as much again for - usually boring - documentation and support tasks.

Companies have exactly the same problem, but they have the option to throw money at the problem. Sure, there are some OS projects with good documentation (usually sponsored by a company) and a lot of proprietary stuff without, but proprietary software usually has more financial backing and that's directly related to good documentation.

I generally agree with you but let me enumerate the FOSS projects that I use and that have good documentation.

Not explicitly company backed: Ruby, Python.

Backed by multiple companies: PostgreSQL, JavaScript.

Backed by one company: Ruby on Rails, Elixir and Phoenix, Nginx.

Don't know: Django, Apache Httpd.

Of course I might be wrong about the categories.

> paying for something doesn't guarantee good documentation.

Isn't that what I said when referring to commercial Linux distros? Of course it would be easy to continue enumerating.

But morally it entitles you much more the complain if you pay and it's poor quality than if you just get it for free with no promises.