You picked two of the best documented projects around in Rust and PostgreSQL. I don't think that inferior documentation for libraries is a foregone conclusion. Rust's serde library is exceptionally well documented IMO.
It's a matter of having the skill and the will though, and I feel like a lot of projects simply don't emphasize documentation. Even so, not everyone's going to be a good technical writer.
What I mean is I think in general languages tend to have better docs.
Vue has excellent docs imo.
But in terms of technical writing being a skill. I agree. I appreciate the effort people put into docs. I try my best but I’m not very good. So I have a lot of respect for anyone who can sit and write excellent docs.
It's been a long time since I've dug for C++ docs but in general I found Qt to be well documented (if a bit disorganized with the newer stuff). STL documentation always seemed inaccessible to me, but then again I'm working off of some pretty hazy memories.
Commercially supported software Qt (and in the parent post, Vue) tend to have good docs. It makes sense: supporting developers is their business model. Likewise tools with ecosystems of companies (postgresql) tend to get good docs over time. The bigger pain points are tools supported by a major company ... but that used by the business rather than sold. There probably are _some_ good docs - especially at the project launch or major milestones - but they aren't as actively curated.
It's a matter of having the skill and the will though, and I feel like a lot of projects simply don't emphasize documentation. Even so, not everyone's going to be a good technical writer.