XCode 5 - Remove offline documentation?
Xcode 5, 6 and 7 documentation itself is located within:

~/Library/Developer/Shared/Documentation/DocSets


You can delete the files and when you re-launch Xcode, it will offer to re-download the current docs if you have automatic updates set.

Before deleting actual documentation, make sure your downloads folder for the images that contain the installers and the documentation are cleaned out:

~/Library/Caches/com.apple.dt.Xcode/Downloads


If you do decide to delete the documentation itself you will no longer have access to it through Xcode — so be careful not to delete the Core DocSets, or any others that you might need access to regularly as you don't fail over gracefully to the web documentation. 

The documentation that is available for download through XCode is not simply a mirror of what Apple shows online. The docs available online amount to about 1/2 of what is actually available from the more comprehensive downloaded versions. Online documentation can also be updated to a newer build of Xcode, so downloading a copy ensures you have the correct documentation for your version of Xcode.