In case you like living on the edge, Dash 4 seems to work great with the macOS Catalina beta.
The Apple API Reference docset was updated to work with the latest docs from the Xcode 11 beta.
To use the Xcode 11 beta docs, make sure
xcode-select -p (in Terminal) points to the location where you have Xcode 11 installed.
xcode-select -s <path-to-xcode> (in Terminal) to change the active version of Xcode and then check for updates in Dash’s Preferences > Downloads to make sure Dash picks up the change.
The Swift docset in Dash is based on swiftdoc.org, which has not been updated to Swift 5 yet. As soon as it’s updated, the docset will also be updated. In the meantime, you can get the Swift 5 docs by using the Apple API Reference docset (mentioned above) with Xcode 11.
Please let me know if you encounter any issues or if you have any special requests.