internal/version: add explanatory comments to runGo

Explain some of the subtle reasons for why runGo behaves as it does.
Do this to reduce chances of something accidentally breaking in the
future.

Based on suggestions Russ made in
https://go-review.googlesource.com/c/dl/+/504037/comment/34895c43_326d1db9/
and in
https://go-review.googlesource.com/c/dl/+/504038/comment/39e3e664_2f21e752/.

Change-Id: I4e8d51e0733d4ca93835906b5103cf98a414fbc5
Reviewed-on: https://go-review.googlesource.com/c/dl/+/743381
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

internal/version/version.go

diff --git a/internal/version/version.go b/internal/version/version.go
index 1876bfe..53f5b6b 100644
--- a/internal/version/version.go
+++ b/internal/version/version.go
@@ -51,9 +51,18 @@
 		log.Fatalf("%s: not downloaded. Run '%s download' to install to %v", version, version, root)
 	}
 
+	// Note: Do not forcibly change GOTOOLCHAIN here, because we want to behave
+	// exactly like go1.X.Y would, and the real Go 1.X.Y would respect the
+	// user's own GOTOOLCHAIN setting, including updating to a newer Go when
+	// necessary.
 	runGo(root, "")
 }
 
+// runGo runs the "go" tool of the Go toolchain whose
+// GOROOT is provided in root. gotoolchain optionally
+// controls the GOTOOLCHAIN environment variable; the
+// empty string means to use any existing GOTOOLCHAIN
+// setting available in the environment as is.
 func runGo(root, gotoolchain string) {
 	gobin := filepath.Join(root, "bin", "go"+exe())
 	cmd := exec.Command(gobin, os.Args[1:]...)
@@ -73,7 +82,14 @@
 	os.Exit(0)
 }
 
+// computeEnv computes the environment of the process in runGo.
+// It's factored out of runGo so that TestComputeEnv can test it.
 func computeEnv(root, gotoolchain string, baseEnv []string) []string {
+	// Put this version's GOROOT/bin first in PATH, so that programs executed
+	// by cmd/go commands like "test", "generate", and "run" can find the same
+	// "go" binary as the one that invoked them. This works around the fact that
+	// some older versions of the go command didn't handle that on their own
+	// (see go.dev/issue/57050 and go.dev/issue/68005).
 	newPath := filepath.Join(root, "bin")
 	if p := os.Getenv("PATH"); p != "" {
 		newPath += string(filepath.ListSeparator) + p