Yes, it supports interfaces and unions. One thing not being stated enough in this thread also is that the docs are not just a regurgitated version of the prototypes -- there's actual hand-written text that tells you what the things return.

In the standard library, it's primarily in the range-based functions that you see this, where the type doesn't generally need to be known. And where you do see it, the documentation will describe what the function returns more accurately than any convoluted made-up type the function signature could show you.