cmd/go: document GOTOOLDIR environment variable

Also, make the variables list sorted.

Fixes #24794

Change-Id: I55f77004b00391875d26df8e55e54d79cef168dc
Reviewed-on: https://go-review.googlesource.com/106255Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -1009,6 +1009,9 @@
 // 		Examples are amd64, 386, arm, ppc64.
 // 	GOBIN
 // 		The directory where 'go install' will install a command.
+// 	GOCACHE
+// 		The directory where the go command will store cached
+// 		information for reuse in future builds.
 // 	GOOS
 // 		The operating system for which to compile code.
 // 		Examples are linux, darwin, windows, netbsd.
@@ -1022,9 +1025,10 @@
 // 	GOTMPDIR
 // 		The directory where the go command will write
 // 		temporary source files, packages, and binaries.
-// 	GOCACHE
-// 		The directory where the go command will store
-// 		cached information for reuse in future builds.
+// 	GOTOOLDIR
+// 		The directory where the go tools (compile, cover, doc, etc...)
+// 		are installed. This is printed by go env, but setting the
+// 		environment variable has no effect.
 //
 // Environment variables for use with cgo:
 //

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

@@ -461,6 +461,9 @@ General-purpose environment variables:
 		Examples are amd64, 386, arm, ppc64.
 	GOBIN
 		The directory where 'go install' will install a command.
+	GOCACHE
+		The directory where the go command will store cached
+		information for reuse in future builds.
 	GOOS
 		The operating system for which to compile code.
 		Examples are linux, darwin, windows, netbsd.
@@ -474,9 +477,10 @@ General-purpose environment variables:
 	GOTMPDIR
 		The directory where the go command will write
 		temporary source files, packages, and binaries.
-	GOCACHE
-		The directory where the go command will store
-		cached information for reuse in future builds.
+	GOTOOLDIR
+		The directory where the go tools (compile, cover, doc, etc...)
+		are installed. This is printed by go env, but setting the
+		environment variable has no effect.
 
 Environment variables for use with cgo: