Quick progress update on this: I just merged a PR that enables Swift-DocC to process Objective-C symbol graph files. The clang portion of the integration that will produce these symbol graph files is still in active development (for example, see PR) and a further update with instructions will be shared once Objective-C doc generation can be used end-to-end. Also, one next step here will be to integrate this in the new DocC SwiftPM plugin.