Requires macOS ≥ 10.9
Please email me at vaclav@slavik.io with any questions or feedback. I’m also @vslavik on Twitter and a good bit of my work is over at GitHub.
Due to sandboxing requirements, Docs cannot just add files to Xcode's directory, you have to give it permission first. Usually, this is just a matter of granting access to the folder the app pre-selects, but sometimes, it doesn't exist yet. This can happen e.g. if Xcode is newly installed and didn't have a chance to download any updates to its own docsets yet.
If this happens, you need to create the folder manually, by running the following shell command from Terminal:
mkdir -p ~/Library/Developer/Shared/Documentation/DocSets
The simplest way is directly from CocoaDocs — hover over the share icon in the top right corner and add to Docs for Xcode. You can also add a feed manually or just open a docset file.