doc/go1.16: add vet release note for CL 235677

For #40700
Fixes #42895

Change-Id: I05b60f0d000512d5dddb3d61e0e695aa01943d6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/274617


Trust: Tim King <taking@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>