I find that self hosting "devdocs" [1] and having zeal (on linux) [2] solves a lot of these problems with the offline docs.
• Info¹ documentation, which I read directly in Emacs. (If you have ever used the terminal-based standalone “info” program, please try to forget all about it. Use Emacs to read Info documentation, and preferably use a graphical Emacs instead of a terminal-based one; Info documentation occasionally has images.)
• Gnome Devhelp².
• Zeal³
• RFC archive⁴ dumps provided by the Debian “doc-rfc“ package⁵.
1. https://www.gnu.org/software/emacs/manual/html_node/info/
2. https://wiki.gnome.org/Apps/Devhelp