In Xcode 16, even if Objective-C is selected, the Navigator in the documentation viewer displays Swift information.
I don't know when this bug was introduced, but it's there in the last Xcode 16 betas at least.
I hope as many developers as possible submit this bug to Apple to get their attention.
P.S. Yes, I know there's Dash. For many years, Dash was a much better option than Xcode's built-in viewer. However, in version 5, the Dash developer introduced some unfortunate UI changes that made Xcode a better option to view the documentation in certain cases. Unfortunately, the Dash developer doesn't seem to be interested in user feedback anymore. He's been ignoring suggestions and user requests for years by now.