According to the "Godoc: documenting Go code" blog post:
URLs will be converted to HTML links; no special markup is necessary.
However, is there a way to create an HTML link with different link text than the URL?
Godoc.org linkifies text like "RFC 1234" to the online RFC (example here).
With that exception, the godoc tool and godoc.org do not provide a way to use text different from the URL.