cmd/go: add release notes for go telemetry, and telemetry env values

cmd/go: add release notes for go telemetry, and telemetry env values

This change fills in the release notes for the go telemetry command as
well as the unsettable GOTELEMETRY and GOTELEMETRYDIR go env values.

For #67111

Change-Id: Id6943f79f7ab2457787e1639d8d5fb1c1e2649dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/587923
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>