I've been playing around with core-plot, a graphing library for MacOS/iOS, which ships with a local dotset. A normal double click on the docset from Finder wasn't enough to read it, after a bit of Googling the solution seems straightforward:
- Shutdown Xcode.
- Copy the .docset in to
- Restart Xcode.
If you want to check that Xcode has picked up the new docset then look in Xcode Preferences under Downloads.