x/tools/cmd/godoc: better output when running with GOMOD=/dev/null

[dmitshur]

When running godoc in module mode while outside of any module (i.e., where go env GOMOD reports /dev/null), godoc displays only the Go packages that are available in the standard library, which is expected per design in #33655 (comment).

However, the current output isn't very user friendly:

$ GO111MODULE=on godoc
using module mode; GOMOD=/dev/null
go mod download: no modules specified (see 'go help mod download')
documentation for some packages is not shown:

It can be improved by detecting when the GOMOD value is /dev/null and treating it differently than other paths. Specifically, there isn't a need to try to fill the module cache in that case.

/cc @bcmills @jayconrod FYI, and please let me know if you notice that I'm overlooking something.

[gopherbot]

Change https://golang.org/cl/206886 mentions this issue: cmd/godoc: don't execute go mod download when GOMOD=/dev/null