On Stack Overflow there is a very similar question on how to obtain the Xcode Documentation as well: Question 1384622
In order to download the documentation (which is compressed in an xar archive) you need to determine where the appropriate files are stored; in this case the appropriate feed is at < http://developer.apple.com/rss/com.apple.adc.documentation.AppleiPhone3%5F1.atom > This can be found by clicking on the i symbol beside the entry in the Documentation section of Xcode's Preferences. You will need to use curl or wget to read the .atom file as Safari won't show the details.
The necessary file needed to be download is http://devimages.apple.com/docsets/Oct2009/com.apple.adc.documentation.AppleiPhone3%5F1.iPhoneLibrary.xar
Then you can follow the instructions from learning2code
Once you've copied the .xar file to your Mac (It's about 125MB) you need to install it:
- Move the .xar file to
/Developer/Documentation/DocSets/ and extract the file by running xar -xf on the file from the same directory. (Run cd /Developer/Documentation/DocSets` first).
- You then need to change the permissions on the files by running
sudo chown -R -P _devdocs *.docset
- Launch Xcode and you should be good to go.