When using the special import "C", the "cgo" build constraint is implied for the go file,
potentially triggering unclear "undefined" error messages.
Explicitly explain this in the documentation.

Updates #24068

Change-Id: Ib656ceccd52c749ffe7fb2d3db9ac144f17abb32
GitHub-Last-Rev: 5a13f00a9b917e51246a5fbb642c4e9ed55aa21d
GitHub-Pull-Request: golang/go#24072
Reviewed-on: https://go-review.googlesource.com/96655Reviewed-by: Ian Lance Taylor <iant@golang.org>