cmd/go: clarify GODEBUG in go help environment

Fixes #37004
Fixes #50444

Change-Id: I7dd5a8c9bd0a2122ff38508cf509369d6d8ad599
Reviewed-on: https://go-review.googlesource.com/c/go/+/632177
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: seankhliao

src/cmd/go/alldocs.go

@@ -511,8 +511,9 @@ General-purpose environment variables:
 		external go command build cache.
 		See 'go doc cmd/go/internal/cacheprog'.
 	GODEBUG
-		Enable various debugging facilities. See https://go.dev/doc/godebug
-		for details.
+		Enable various debugging facilities for programs built with Go,
+		including the go command. Cannot be set using 'go env -w'.
+		See https://go.dev/doc/godebug for details.
 	GOENV
 		The location of the Go environment configuration file.
 		Cannot be set using 'go env -w'.