doc: document cmd/vet changes for 1.18 release

cmd/vet has several precision improvements for the checkers copylock, printf, sortslice, testinggoroutine, and tests. Adds a high level mention in the release notes and an example of string constant concatenation.

Updates #47694

Change-Id: I7a342a57ca3fd9e2f3e8ec99f7b647269798317f
Reviewed-on: https://go-review.googlesource.com/c/go/+/370734


Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Tim King <taking@google.com>
Run-TryBot: Tim King <taking@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>