Hi @nathan!
Thanks for posting this. You're correct; today Swift-DocC does not support linking to symbols defined in other frameworks.
We're currently tracking adding support with [SR-15431] Support DocC references to symbols defined in another module · Issue #208 · apple/swift-docc · GitHub. If you could add some of the details from your post here in that bug report so we can be sure to cover your use case, that would be really helpful!