runtime: clarify GOROOT return value in documentation

The current GOROOT documentation could indicate that changing the
environment variable at runtime would affect the return value of
GOROOT. This is false as the returned value is the one used for the
build. This CL aims to clarify the confusion.

Fixes #22302

Change-Id: Ib68c30567ac864f152d2da31f001a98531fc9757
Reviewed-on: https://go-review.googlesource.com/75751Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/extern.go

@@ -212,8 +212,8 @@ func Callers(skip int, pc []uintptr) int {
 	return callers(skip, pc)
 }
 
-// GOROOT returns the root of the Go tree.
-// It uses the GOROOT environment variable, if set,
+// GOROOT returns the root of the Go tree. It uses the
+// GOROOT environment variable, if set at process start,
 // or else the root used during the Go build.
 func GOROOT() string {
 	s := gogetenv("GOROOT")