I can't really comment from experience, as I made my first ever open source pull requests only last night 😃, but I will make and practical observation:

Open source documentation is often extremely good. I guess it's because the contributors want to be understood and to welcome more people to contribute. In closed source corporations, everything is kept secret because individual employees don't want others to understand. Knowledge is power, and hence they don't share their knowledge

I feel that this attitude is generally better in open source.