doc: add release note on coverage testing of no-test packages

doc: add release note on coverage testing of no-test packages

Add a small release note blurb to describe the changes in CL 495447
relating to "go test -cover" runs on packages with functions but no
tests.

For #61422.

Change-Id: Ib8163ac70b902f0d7f9f470b944e7f70711e3cf3
Reviewed-on: https://go-review.googlesource.com/c/go/+/547635
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>