Document the CSS class used for the ScaleButton popover

Otherwise it's not easy to divine the selector for the scale widget
inside the popover.
diff --git a/gtk/gtkscalebutton.c b/gtk/gtkscalebutton.c
index e74dcf2..82829b1 100644
--- a/gtk/gtkscalebutton.c
+++ b/gtk/gtkscalebutton.c
@@ -70,6 +70,8 @@
  *
  * GtkScaleButton has a single CSS node with name button. To differentiate
  * it from a plain #GtkButton, it gets the .scale style class.
+ *
+ * The popup widget that contains the scale has a .scale-popup style class.
  */