cmd/go: document unsettable GOTELEMETRY and GOTELEMETRYDIR go env vars

This change adds documentation to "go help telemetry" and "go help
environment" for the unsettable GOTELEMETRY and GOTELEMETRYDIR go env
variables.

For #68928

Change-Id: I8c139bd8585ffb0c45b9dd722ef6e7a9a33df192
Reviewed-on: https://go-review.googlesource.com/c/go/+/607855
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: matloob

src/cmd/go/alldocs.go

@@ -678,6 +678,11 @@ Additional information available from 'go env' but not read from the environment
 		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.
+	GOTELEMETRY
+		The current Go telemetry mode ("off", "local", or "on").
+		See "go help telemetry" for more information.
+	GOTELEMETRYDIR
+		The directory Go telemetry data is written is written to.
 	GOTOOLDIR
 		The directory where the go tools (compile, cover, doc, etc...) are installed.
 	GOVERSION

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

@@ -41,6 +41,11 @@ To disable telemetry uploading, but keep local data collection, run
 To enable both collection and uploading, run “go telemetry on”.
 To disable both collection and uploading, run "go telemetry off".
 
+The current telemetry mode is also available as the value of the
+non-settable "GOTELEMETRY" go env variable. The directory in the
+local file system that telemetry data is written to is available
+as the value of the non-settable "GOTELEMETRYDIR" go env variable.
+
 See https://go.dev/doc/telemetry for more information on telemetry.
 `,
 	Run: runTelemetry,