[introspection] Merge in Gtk-custom.c annotations

The Gtk-custom.c file in gir-repository contained a number of
introspection annotations.  Merge those into the GTK source files.

Some documentation was moved from the tmpl/ files to accomodate
the addition of annotations.

gtk/gtkprintbackend.c

@@ -304,6 +304,11 @@ _gtk_print_backend_create (const gchar *backend_name)
   return pb;
 }
 
+/**
+ * gtk_printer_backend_load_modules:
+ *
+ * Return value: (element-type GtkPrintBackend) (transfer container):
+ */
 GList *
 gtk_print_backend_load_modules (void)
 {
@@ -587,6 +592,11 @@ gtk_print_backend_set_list_done (GtkPrintBackend *backend)
 }
 
 
+/**
+ * gtk_print_backend_get_printer_list:
+ *
+ * Return value: (element-type GtkPrinter) (transfer container):
+ */
 GList *
 gtk_print_backend_get_printer_list (GtkPrintBackend *backend)
 {