how to add the existed iOS DocSet to xcode?

Question

I have already downloaded the iOS 6.1 DocSet, as shown here in Finder when I navigate to /Applications/Xcode.app/Contents/Developer/Documentation/DocSets/:

But, when in Xcode preferences it prompts me to Install it again, even though it's already been downloaded:

How can I make Xcode read the already installed DocSet?