cmd/go: documentation doesn't mention GOENV=off

[mdempsky]

The documentation at https://golang.org/cmd/go/#hdr-Environment_variables doesn't mention that GOENV=off is a special case for disabling the use of a Go environment configuration file. It probably should, and maybe to mention GOENV=./off is a workaround if they actually want to use a file named "off" in the current directory.

[ianlancetaylor]

Perhaps also worth noting that

GOENV=off go env GOENV

just prints a newline. It does not print off. I can't tell whether this is a bug or not.