There is probably no Operating System that is documented more and better than OpenBSD. Every single binary, library, driver or script delivered in the base system has its own manpage.
The OpenBSD project considers lack of documentation on any function of the system to be a bug.
man cat # of course, base system binary
man vio # the virtual network driver
man null # the /dev/null device
man daily # the cronjobs that maintain your system
man adventure # a game delivered in the base system
man hostname.if # network config documentation