The two largest projects that I write documentation for are https://docs.datasette.io/ and https://sqlite-utils.datasette.io/ - they're both big projects with multiple contributors, though most of the commits are still from me.
I find the same approach I take to READMEs for smaller projects scales up pretty well: any time I make a change to one of my larger projects I ensure that the documentation is updated as part of that change.
If I accept a PR from someone without documentation I'll follow up by adding the docs for it myself in the next commit. I think that's more reasonable than demanding people add documentation if it's not necessarily their core skill set.
Honestly, I'd love to experience working with a professional technical writer on this kind of thing, but that's not something that's happened at any point in my career to date!
Writing non-code is easy and fun to me - I used to do it for a living - but when I am pressed to deliver features, documentation takes a backseat. Also, I've never felt like documenting other people's code