cmd/go: mention GODEBUG in "go help environment"

Change-Id: Id2c07ccba7c514ec3a4bbd493edbc68ae5cd6250
Reviewed-on: https://go-review.googlesource.com/c/go/+/187157
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/alldocs.go

@@ -1575,6 +1575,9 @@
 // 	GOCACHE
 // 		The directory where the go command will store cached
 // 		information for reuse in future builds.
+// 	GODEBUG
+// 		Enable various debugging facilities. See 'go doc runtime'
+// 		for details.
 // 	GOENV
 // 		The location of the Go environment configuration file.
 // 		Cannot be set using 'go env -w'.

src/cmd/go/internal/help/helpdoc.go

@@ -493,6 +493,9 @@ General-purpose environment variables:
 	GOCACHE
 		The directory where the go command will store cached
 		information for reuse in future builds.
+	GODEBUG
+		Enable various debugging facilities. See 'go doc runtime'
+		for details.
 	GOENV
 		The location of the Go environment configuration file.
 		Cannot be set using 'go env -w'.