[gtk/wip/exalm/actionbar: 2/3] actionbar: Fix the css node docs

[Alexander Mikhaylenko <alexm src gnome org>]

commit 2c8e4573f2ccf4b3b4944fc46b991f43e3d8c547
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Sun May 9 16:55:48 2021 +0500

    actionbar: Fix the css node docs

 gtk/gtkactionbar.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)
---
diff --git a/gtk/gtkactionbar.c b/gtk/gtkactionbar.c
index e82ed3d7c0..7bbcddfae8 100644
--- a/gtk/gtkactionbar.c
+++ b/gtk/gtkactionbar.c
@@ -49,7 +49,23 @@
  *
  * # CSS nodes
  *
- * `GtkActionBar` has a single CSS node with name actionbar.
+ * ```
+ * actionbar
+ * ╰── revealer
+ *     ╰── box
+ *         ├── box.start
+ *         │   ╰── [start children]
+ *         ├── [center widget]
+ *         ╰── box.end
+ *             ╰── [end children]
+ * ```
+ *
+ * A `GtkActionBar`'s CSS node is called `actionbar`. It contains a `revealer`
+ * subnode, which contains a `box` subnode, which contains two `box` subnodes at
+ * the start and end of the action bar, with `start` and `end style classes
+ * respectively, as well as a center node that represents the center child.
+ *
+ * Each of the boxes contains children packed for that side.
  */
 
 typedef struct _GtkActionBarClass         GtkActionBarClass;