[gtk] ListBox: Document buildable child type placeholder

[Daniel Boles <dboles src gnome org>]

commit 217f9ea3b82a038d4ef1baeeb84a0800486ff1db
Author: Daniel Boles <dboles src gmail com>
Date:   Tue Jun 18 21:43:07 2019 +0100

    ListBox: Document buildable child type placeholder
    
    andyholmes on IRC asked about this and it wasn't documented, so fix that

 gtk/gtklistbox.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/gtk/gtklistbox.c b/gtk/gtklistbox.c
index 44743bfdae..0f3a42a931 100644
--- a/gtk/gtklistbox.c
+++ b/gtk/gtklistbox.c
@@ -70,6 +70,12 @@
  *
  * The GtkListBox widget was added in GTK+ 3.10.
  *
+ * # GtkListBox as GtkBuildable
+ *
+ * The GtkListBox implementation of the #GtkBuildable interface supports
+ * setting a child as the placeholder by specifying “placeholder” as the “type”
+ * attribute of a <child> element. See gtk_list_box_set_placeholder() for info.
+ *
  * # CSS nodes
  *
  * |[<!-- language="plain" -->