cmd/cgo: mention //go:build line in godoc

cmd/cgo: mention //go:build line in godoc

Say "//go:build" instead of "// +build" in the package level godoc
comment.

Change-Id: I4700227a03197ffbe29e4de04d068b4c63bb5bf0
Reviewed-on: https://go-review.googlesource.com/c/go/+/431856
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>