cmd/doc: add type-bound vars to global vars list

Already done for constants and funcs, but I didn't realize that some
global vars were also not in the global list. This fixes

	go doc build.Default

Change-Id: I768bde13a400259df3e46dddc9f58c8f0e993c72
Reviewed-on: https://go-review.googlesource.com/9764Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/doc/pkg.go

@@ -71,6 +71,7 @@ func parsePackage(pkg *build.Package, userPath string) *Package {
 	docPkg := doc.New(astPkg, pkg.ImportPath, doc.AllDecls)
 	for _, typ := range docPkg.Types {
 		docPkg.Consts = append(docPkg.Consts, typ.Consts...)
+		docPkg.Vars = append(docPkg.Vars, typ.Vars...)
 		docPkg.Funcs = append(docPkg.Funcs, typ.Funcs...)
 	}