[gtk+] docs: Add a diagram for GtkListBox nodes

[Emmanuele Bassi <ebassi src gnome org>]

commit b74cd23e299418d67114c63ca4bc66dcf5af47e1
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Tue Sep 20 23:32:00 2016 +0100

    docs: Add a diagram for GtkListBox nodes
    
    GtkListBox's API reference for CSS nodes should follow the template of
    other widget classes.

 gtk/gtklistbox.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtklistbox.c b/gtk/gtklistbox.c
index 3adb904..0269115 100644
--- a/gtk/gtklistbox.c
+++ b/gtk/gtklistbox.c
@@ -65,8 +65,13 @@
  *
  * # CSS nodes
  *
- * GtkListBox uses a single CSS node with name list. GtkListBoxRow uses
- * a single CSS node with name row. The row nodes get the .activatable
+ * |[<!-- language="plain" -->
+ * list
+ * ╰── row[.activatable]
+ * ]|
+ *
+ * GtkListBox uses a single CSS node named list. Each GtkListBoxRow uses
+ * a single CSS node named row. The row nodes get the .activatable
  * style class added when appropriate.
  */