[gtk+] spin button: Add diagrams to CSS documentation

[Matthias Clasen <matthiasc src gnome org>]

commit 65d959bbc8aa24a97c5dbb56896a2d724c741151
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Nov 3 13:36:26 2015 -0500

    spin button: Add diagrams to CSS documentation

 gtk/gtkspinbutton.c |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkspinbutton.c b/gtk/gtkspinbutton.c
index 9771695..7375b5f 100644
--- a/gtk/gtkspinbutton.c
+++ b/gtk/gtkspinbutton.c
@@ -82,6 +82,22 @@
  *
  * # CSS nodes
  *
+ * |[<!-- language="plain" -->
+ * spinbutton.horizontal
+ * ├── entry
+ * │   ╰── ...
+ * ├── button.down
+ * ╰── button.up
+ * ]|
+ *
+ * |[<!-- language="plain" -->
+ * spinbutton.vertical
+ * ├── button.up
+ * ├── entry
+ * │   ╰── ...
+ * ╰── button.down
+ * ]|
+ *
  * GtkSpinButtons main CSS node has the name spinbutton. It creates subnodes
  * for the entry and the two buttons, with these names. The button nodes have
  * the style classes .up and .down. The GtkEntry subnodes (if present) are put