combo box: Add diagrams to CSS documentation

gtk/gtkcombobox.c

@@ -90,8 +90,13 @@
  *
  * # CSS nodes
  *
+ * |[<!-- language="plain" -->
+ * combobox
+ * ╰── button.combo
+ * ]|
+ *
  * GtkComboBox has a single CSS node with name combobox. It adds the
- * .combo style class to the button (and entry) that it contains.
+ * .combo style class to the button that it contains.
  */
 
 

gtk/gtkcomboboxtext.c

@@ -70,6 +70,18 @@
  *   </items>
  * </object>
  * ]|
+ *
+ * # CSS nodes
+ *
+ * |[<!-- language="plain" -->
+ * combobox
+ * ├── entry.combo
+ * ╰── button.combo
+ * ]|
+ *
+ * GtkComboBoxText has a single CSS node with name combobox. It adds
+ * the style class .combo to the main CSS nodes of its entry and button
+ * children.
  */
 
 static void     gtk_combo_box_text_buildable_interface_init     (GtkBuildableIface *iface);