cmd/go/internal/modfetch: document DownloadDir

Change-Id: I4717964234fca0c8c5889ed710b66f39eb53a809
Reviewed-on: https://go-review.googlesource.com/c/151562
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/modfetch/cache.go

@@ -56,6 +56,8 @@ func CachePath(m module.Version, suffix string) (string, error) {
 	return filepath.Join(dir, encVer+"."+suffix), nil
 }
 
+// DownloadDir returns the directory to which m should be downloaded.
+// Note that the directory may not yet exist.
 func DownloadDir(m module.Version) (string, error) {
 	if PkgMod == "" {
 		return "", fmt.Errorf("internal error: modfetch.PkgMod not set")