[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 129bb1917b4914f0743ec9b4ef0dfb74df39c07d)
    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>