December 07, 2011
As a part of following the Stanford iPhone Development course I upgraded my XCode installation to the latest version, 4.2.1 (Build 4D502). However after the update I wasn't able to access any of the documentation.
Turns out it isn't automatically downloaded and installed for you. To set it up do this:
After the documentation downloads, hold down the option key and single-click objects, classes, or methods to see popup documentation window, or option-double-click to open the XCode Organizer Documentation tab, with a full listing about the selected item.