[gtk+] notebook: Update CSS node docs

[Matthias Clasen <matthiasc src gnome org>]

commit 990be25d6366849d6fc32acfa02c19c4a65a26b4
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Jan 11 13:55:23 2016 -0500

    notebook: Update CSS node docs
    
    The stack node is new.

 gtk/gtknotebook.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/gtk/gtknotebook.c b/gtk/gtknotebook.c
index 8f1366f..825ffd6 100644
--- a/gtk/gtknotebook.c
+++ b/gtk/gtknotebook.c
@@ -115,9 +115,10 @@
  * │   │   ╰── [arrow]
  * │   ╰── [<action widget>]
  * │
- * ├── <child>
- * ┊
- * ╰── <child>
+ * ╰── stack
+ *     ├── <child>
+ *     ┊
+ *     ╰── <child>
  * ]|
  *
  * GtkNotebook has a main CSS node with name notebook, a subnode