cmd/go: add documentation for the -overlay flag

Also add -overlay to the Go 1.16 release notes.

For #40700
Fixes #39958
Fixes #42893

Change-Id: Ifd397549e368b255e7b8800986cfa0563a942af5
Reviewed-on: https://go-review.googlesource.com/c/go/+/274714


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>