ListBox: Document buildable child type placeholder

andyholmes on IRC asked about this and it wasn't documented, so fix that

diff --git a/gtk/gtklistbox.c b/gtk/gtklistbox.c
index 44743bfdae8cfc962b90c1684a17afebaea11e98..0f3a42a931d0c5327a58e75707f1198d6392cc06 100644 (file)
--- 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" -->