cmd/go: document 'go env GOMOD' outside modules

In documentation for 'go env GOMOD', note that the path will be
os.DevNull in module-aware mode when no go.mod file is present.

Fixes #36052

Change-Id: I30ced1df02ccefe1970bd856190e79d6f0384375
Reviewed-on: https://go-review.googlesource.com/c/go/+/210577
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -1726,8 +1726,10 @@
 // 	GOHOSTOS
 // 		The operating system (GOOS) of the Go toolchain binaries.
 // 	GOMOD
-// 		The absolute path to the go.mod of the main module,
-// 		or the empty string if not using modules.
+// 		The absolute path to the go.mod of the main module.
+// 		If module-aware mode is enabled, but there is no go.mod, GOMOD will be
+// 		os.DevNull ("/dev/null" on Unix-like systems, "NUL" on Windows).
+// 		If module-aware mode is disabled, GOMOD will be the empty string.
 // 	GOTOOLDIR
 // 		The directory where the go tools (compile, cover, doc, etc...) are installed.
 //

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

@@ -621,8 +621,10 @@ Additional information available from 'go env' but not read from the environment
 	GOHOSTOS
 		The operating system (GOOS) of the Go toolchain binaries.
 	GOMOD
-		The absolute path to the go.mod of the main module,
-		or the empty string if not using modules.
+		The absolute path to the go.mod of the main module.
+		If module-aware mode is enabled, but there is no go.mod, GOMOD will be
+		os.DevNull ("/dev/null" on Unix-like systems, "NUL" on Windows).
+		If module-aware mode is disabled, GOMOD will be the empty string.
 	GOTOOLDIR
 		The directory where the go tools (compile, cover, doc, etc...) are installed.
 	`,