Docs question on building with mage and go modules

I’m setting up a new computer, so I went through the docs on setting up a new environment for building/contributing. The interaction with Mage and go modules has left me a bit puzzled because the development docs don’t mention Mage but the file does.

Mage doesn’t seem to work when I have the Hugo repo in my GOPATH. go build seems to work either way. Is that something peculiar to my setup (go 1.11.4) or is that expected? If that’s expected, should I update the development docs,, or both?