cmd/go: document import "C" check from CL 129062
cmd/go: document import "C" check from CL 129062

Added this locally but then broke the first rule of Gerrit
and clicked Submit instead of running "git submit".

Change-Id: I83c28d9151c566e9b2092e2613d67731a5d64beb
Reviewed-on: https://go-review.googlesource.com/129678
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/go,nawawi/go,josharian/go,cloudflare/go,golang/go,akutz/go,golang/go,josharian/go,josharian/go,nawawi/go,josharian/go,josharian/go,josharian/go,akutz/go,cloudflare/go,akutz/go,nawawi/go,josharian/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,golang/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go