I was recently on a mission to clean up the documentation sets that are installed on my system, specifically, OS 3.x docs that I no longer need. The obvious place to look was within the Documentation tab of the Preferences settings. It was the right idea, however, I discovered the “-” option to remove a docset is only available if the documentation has not been installed.
To remove a documentation begin by clicking the arrow enclosed in the box on the lower left corner (shown in the red box in the Figure above).
A window will expand showing additional information about the documentation. The Installed Location value is a link showing where on your system the documentation is installed.
If you click on the link, Finder will open showing the folder with the API document.
Delete the document in Finder and return to Xcode, you’ll now see the doc set removed from the list.