Dash is an offline documentation managemer on OSX
It’s free and has limitation, but you can purchase it and get all function for $19.9.
We always need write code anywhere, anytime, but sometimes there is no network connection. With Dash you can download docset and manage it, and have an exhaustive doc search engine in the computer for offline use.
It also can integrate in many text editor like SublimeText, Vim, emacs etc.