cmd/cgo: readability improvements to generated _cgo_export.h

Also copy doc comments from Go code to _cgo_export.h.

This is a step toward installing this generated file when using
-buildmode=c-archive or c-shared, so that C code can #include it.

Change-Id: I3a243f7b386b58ec5c5ddb9a246bb9f9eddc5fb8
Reviewed-on: https://go-review.googlesource.com/9790
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/cgo/doc.go

@@ -239,6 +239,9 @@ The following options are available when running cgo directly:
 		syscall package when bootstrapping a new target.
 	-objdir directory
 		Put all generated files in directory.
+	-importpath string
+		The import path for the Go package. Optional; used for
+		nicer comments in the generated files.
 	-gccgo
 		Generate output for the gccgo compiler rather than the
 		gc compiler.