I usually make do with `man` pages for C (very comprehensive), and the `python-docs` package in my distro (you can download a static copy of the python documentation, and even the search will work offline). As well as whatever PDF manuals I need for what I'm doing.

As I use Gentoo I also usually have the source code for anything on my system, so I can dig into that if I'm missing some docs, as there's frequently a docs/ folder in the archive.

If I'm missing documentation, I make a note of it and see if there's a way to make it available locally, somehow.

https://zealdocs.org/ is surprisingly decent.