The initial Swift-DocC and Swift-DocC Render changes are already available in recent Swift 6.0 toolchains.
Additionally, as we were working on this and using it we realized that we also wanted to land Synthesize a minimal landing page for combined archives by d-ronnqvist · Pull Request #1011 · swiftlang/swift-docc · GitHub for as an important enhancement and Support `--output-path` when building documentation for multiple targets. by d-ronnqvist · Pull Request #89 · swiftlang/swift-docc-plugin · GitHub as a quality-of-life improvement.
The former is available in recent Swift development toolchains and all Swift-DocC Plugin changes are available in the 1.4.x release.
With those changes, we feel like there's enough pieces in place for people to try out an early preview of what's currently available. I made a separate post about this just now.
Because there may still be breaking changes we've not updated the main package documentation to cover this yet. However, the plugin 1.4 release notes and the other forums thread from just now both show minimal examples of using the new flag.
In short, it should just be "build some number of targets/products" and pass the --enable-experimental-combined-documentation flag. You don't need to pass any extra feature flags to DocC. The plugin takes care of that.