scrolledwindow: Fix code sample compilation

gtk/gtkscrolledwindow.c

@@ -70,8 +70,13 @@
  *
  * If gtk_container_add() has added a #GtkViewport for you, you can remove
  * both your added child widget from the #GtkViewport, and the #GtkViewport
- * from the GtkScrolledWindow, with either of these calls:
+ * from the GtkScrolledWindow, like this:
  * |[<!-- language="C" -->
+ * GtkWidget *scrolled_window = gtk_scrolled_window_new (NULL, NULL);
+ * GtkWidget *child_widget = gtk_button_new ();
+ *
+ * // GtkButton is not a GtkScrollable, so GtkScrolledWindow will automatically
+ * // add a GtkViewport.
  * gtk_container_remove (GTK_CONTAINER (scrolled_window),
  *                       child_widget);
  * // or