[gtk/doc-scale-button-css] Document the CSS class used for the ScaleButton popover

[Emmanuele Bassi <ebassi src gnome org>]

commit e7bd9adf3ed585f3b7492763a9ea7d8f2ba2c854
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Fri Dec 13 11:27:22 2019 +0000

    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.

 gtk/gtkscalebutton.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gtk/gtkscalebutton.c b/gtk/gtkscalebutton.c
index e74dcf2b49..82829b153f 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.
  */