cmd/go: document purego convention

cmd/go: document purego convention

Fixes #23172

Change-Id: I854e399471dfa22e62fbdec9719e561c4501e5ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/660136
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>