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 <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>