A prior install of a XCode7-beta was inadvertently cancelled during the download in progress of the iOS documentation (via Preferences > Downloads) causing it to be falsely checked as "installed" there. However, use of either Quick Help or Documentation and API Reference DOES NOT access the iOS9 documentation (e.g. the unwindForSegue method is not there), instead only the iOS 8.x documentation is accessible (from previous XCode 6.4 install?)
I've now installed the (registered developer) release version of XCode 7.0.1 over the prior betas, hoping what behaves like a stale-link/cache problem would be cleared up- it's not. Again Preferences > Downloads is checked for iOS9 Documentation indicating it's installed. Opening the Xcode app indicates the file exists com.apple.adc.docuForSeguementation.iOS.docset with a date of August 31, 2015 9:27 PM (corresponding to XCode 7.0.1?), but use of either Quick Help or Documentation and API Reference yields results (no errors mind you) consistent with iOS 8.x.
I've deleted and reinstalled XCode 7.0.1 with the same maddening results.