cmd/cgo: applied alternative wording to cgo constraint

A change suggested by @ianlancetaylor on gerrit, provides a more direct and clear message for the documentation.

Author: ikkerens

src/cmd/cgo/doc.go

@@ -109,9 +109,11 @@ it is expected to work. It is disabled by default when
 cross-compiling. You can control this by setting the CGO_ENABLED
 environment variable when running the go tool: set it to 1 to enable
 the use of cgo, and to 0 to disable it. The go tool will set the
-build constraint "cgo" if cgo is enabled. When the Go tool sees the
-special import "C", the "cgo" build constraint will be implied, causing
-the tool to skip the file if CGO_ENABLED is set to 0.
+build constraint "cgo" if cgo is enabled. The special import "C"
+implies the "cgo" build constraint, as though the file also said
+"// +build cgo".  Therefore, if cgo is disabled, files that import
+"C" will not be built by the go tool. 
+(For more about build constraints see https://golang.org/pkg/go/build/#hdr-Build_Constraints).
 
 When cross-compiling, you must specify a C cross-compiler for cgo to
 use. You can do this by setting the generic CC_FOR_TARGET or the