[release-branch.go1.15] doc/go1.15: mention 1.15.3 cgo restriction on empty structs

For #40954

Change-Id: I6a30aed31a16e820817f4ca5c7f591222e922946
Reviewed-on: https://go-review.googlesource.com/c/go/+/277432


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit 129bb191)
Reviewed-on: https://go-review.googlesource.com/c/go/+/278573


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>