[gtk/wip/exalm/actionbar] actionbar: Document GtkBuildable child types

[Alexander Mikhaylenko <alexm src gnome org>]

commit d42c51e27d0ce40e08381d5997c587c258eae1a8
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Sun Jul 3 02:26:18 2022 +0400

    actionbar: Document GtkBuildable child types

 gtk/gtkactionbar.c | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/gtk/gtkactionbar.c b/gtk/gtkactionbar.c
index 9d7b312683..5405315029 100644
--- a/gtk/gtkactionbar.c
+++ b/gtk/gtkactionbar.c
@@ -47,6 +47,13 @@
  * the full width of the box, even if the children at either side take
  * up different amounts of space.
  *
+ * # GtkActionBar as GtkBuildable
+ *
+ * The `GtkActionBar` implementation of the `GtkBuildable` interface supports
+ * adding children at the start or end sides by specifying “start” or “end” as
+ * the “type” attribute of a `<child>` element, or setting the center widget
+ * by specifying “center” value.
+ *
  * # CSS nodes
  *
  * ```