Merge branch 'doc-scale-button-css' into 'gtk-3-24'

Document the CSS class used for the ScaleButton popover

See merge request !1246

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.
  */