DocC currently does not support references to symbols/types defined in other modules, including symbols defined as extensions in the current module.
@Mordil is correct here. Although, I would separate those as two separate issues. Linking to symbols defined in other modules is a new feature we would would definitely like to support in Swift-DocC (@Mordil a feature request on bugs.swift.org would be great!), while I would view the behavior @robnik described as a bug. Those symbols are defined in your current module, so DocC should support them even without the cross-module linking feature.
@robnik if you could file a bug on bugs.swift.org with the behavior you’re seeing as well as your proposed solution for link syntax that would be really appreciated! One of things we need to work through here is the UX of how extension symbols would be rendered and linked to.