Languages from the GNU project tend to have decent info pages (e.g. Guile, GCL, Libc). Perl also comes with perldoc, which is like manpages but instead of using the man command you use the perldoc command.
In python you can run help() on everything as I recently discovered.
I wanted to say C++ but I noticed decent
cppreference is a great doc and can be download
True, I remember downloading it to my laptop before going to the South Pole.