firestore: document that a ts.Timestamp can be used when saving data.
@@ -96,7 +96,8 @@ func (d *DocumentRef) Get(ctx context.Context) (*DocumentSnapshot, error) {
// is the underlying type of a Integer.
// - float32 and float64 convert to Double.
// - []byte converts to Bytes.
-// - time.Time converts to Timestamp.
+// - time.Time and *ts.Timestamp convert to Timestamp. ts is the package
+// "github.com/golang/protobuf/ptypes/timestamp".
// - *latlng.LatLng converts to GeoPoint. latlng is the package
// "google.golang.org/genproto/googleapis/type/latlng". You should always use
// a pointer to a LatLng.
