Good resources for offline documentation to code without internet?

I was on a road trip recently, and decided to try learning Vulkan while I was in the passenger seat, as I had downloaded a PDF of a Vulkan tutorial before I left. I wrote most of a Makefile for my project, but found myself struggling to remember specific automatic variables. I tried `man make`, but that only described the usage of the program itself, not writing Makefiles. Additionally, I found that I had no reference for SDL2, and resorted to reading header files to glean specific API details.

I've found myself in this situation before, trying to learn new things on long flights as well.

Even if there were man pages for some of these things, HTML documentation might be preferable. It would be nice to have a program or library that could download language and project specific documentation in an easily searchable and browsable library. Does anything like this currently exist?