Open: https://developer.apple.com/library/downloads/docset-index.dvtdownloadableindex.
Find your entry by name like ‘Xcode 7.3 Documentation’ listed in Xcode, and find out the latest among three or more with the same name.
Then download the dmg and install.
Download Xcode Documentations by @sskaje: https://sskaje.me/2016/07/download-xcode-documentations/
Link to this post!