doc: document generalized function type inference (update release notes)

For #77245.

Change-Id: If36098fc36290fe2430d8ef996f35100f8113739
Reviewed-on: https://go-review.googlesource.com/c/go/+/783362
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
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: Dmitri Shuralyov <dmitshur@golang.org>