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

[Emmanuele Bassi <ebassi src gnome org>]

commit 365cc08c9c37e51a72249f4d96fb4da4d4898c66
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 | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkscalebutton.c b/gtk/gtkscalebutton.c
index e74dcf2b49..1977988fa5 100644
--- a/gtk/gtkscalebutton.c
+++ b/gtk/gtkscalebutton.c
@@ -68,8 +68,10 @@
  *
  * # CSS nodes
  *
- * GtkScaleButton has a single CSS node with name button. To differentiate
- * it from a plain #GtkButton, it gets the .scale style class.
+ * 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.
  */