[gtk/matthiasc/for-master] menubutton: Correct the css docs

[Matthias Clasen <matthiasc src gnome org>]

commit b0b793dd949b12b38eccc0e89a2923cb193d139e
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Dec 23 08:45:51 2020 -0500

    menubutton: Correct the css docs
    
    We are using menubutton instead of button.popup now.

 gtk/gtkmenubutton.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkmenubutton.c b/gtk/gtkmenubutton.c
index d37ee47fc1..29a320aff3 100644
--- a/gtk/gtkmenubutton.c
+++ b/gtk/gtkmenubutton.c
@@ -104,8 +104,14 @@
  *
  * # CSS nodes
  *
- * GtkMenuButton has a single CSS node with name button. To differentiate
- * it from a plain #GtkButton, it gets the .popup style class.
+ * |[<!-- language="plain" -->
+ * menubutton
+ * ╰── button.toggle
+ *     ╰── [content]
+ *]|
+ *
+ * GtkMenuButton has a single CSS node with name menubutton
+ * which contains a toggle button node.
  *
  * # Accessibility
  *