doc: mention stack allocation of slices

doc: mention stack allocation of slices

Very similar to last release's note.

Change-Id: Ie7afe21d98cee1c9718e53b20e8af8ee18504bb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/725921
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>