doc: document new struct literal field selectors (update release notes)

For #9859.

Change-Id: Ib39e655fa603345f661e0b32bb56fd532854103c
Reviewed-on: https://go-review.googlesource.com/c/go/+/783400
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>