cmd/go: document that functions are exported by cgo

The buildmode docs mention exported functions, but don't say anything
about how to export them.  Mention the cgo tool to make this somewhat
clearer.

Fixes #11955.

Change-Id: Ie5420445daa87f5aceec6ad743465d5d32d0a786
Reviewed-on: https://go-review.googlesource.com/13080Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/alldocs.go

@@ -781,14 +781,14 @@ are:
 	-buildmode=c-archive
 		Build the listed main package, plus all packages it imports,
 		into a C archive file. The only callable symbols will be those
-		functions marked as exported. Requires exactly one main package
-		to be listed.
+		functions marked as exported by the cgo tool. Requires exactly
+		one main package to be listed.
 
 	-buildmode=c-shared
 		Build the listed main packages, plus all packages that they
 		import, into C shared libraries. The only callable symbols will
-		be those functions marked as exported. Non-main packages are
-		ignored.
+		be those functions marked as exported by the cgo tool. Non-main
+		packages are ignored.
 
 	-buildmode=default
 		Listed main packages are built into executables and listed

src/cmd/go/help.go

@@ -548,14 +548,14 @@ are:
 	-buildmode=c-archive
 		Build the listed main package, plus all packages it imports,
 		into a C archive file. The only callable symbols will be those
-		functions marked as exported. Requires exactly one main package
-		to be listed.
+		functions marked as exported by the cgo tool. Requires exactly
+		one main package to be listed.
 
 	-buildmode=c-shared
 		Build the listed main packages, plus all packages that they
 		import, into C shared libraries. The only callable symbols will
-		be those functions marked as exported. Non-main packages are
-		ignored.
+		be those functions marked as exported by the cgo tool. Non-main
+		packages are ignored.
 
 	-buildmode=default
 		Listed main packages are built into executables and listed