goose: require pointer for first argument to goose.Bind (google/go-cloud#31)

Fixes google/go-cloud#15

goose.go

@@ -43,11 +43,12 @@ func Build(...interface{}) string {
 type Binding struct{}
 
 // Bind declares that a concrete type should be used to satisfy a
-// dependency on iface.
+// dependency on the type of iface, which must be a pointer to an
+// interface type.
 //
 // Example:
 //
-//	var MySet = goose.NewSet(goose.Bind(MyInterface(nil), new(MyStruct)))
+//	var MySet = goose.NewSet(goose.Bind(new(MyInterface), new(MyStruct)))
 func Bind(iface, to interface{}) Binding {
 	return Binding{}
 }