GtkApplication: Clarify docs a bit

Explicitly mention how gtk_application_set_accels_for_action can
be used to remove all accelerators for an action.

https://bugzilla.gnome.org/show_bug.cgi?id=729451

gtk/gtkapplication.c

@@ -1483,10 +1483,13 @@ normalise_detailed_name (const gchar *detailed_action_name)
  * @accels: (array zero-terminated=1): a list of accelerators in the format understood by
  *     gtk_accelerator_parse()
  *
- * Sets one or more keyboard accelerator that will trigger the
+ * Sets zero or more keyboard accelerators that will trigger the
  * given action. The first item in @accels will be the primary 
  * accelerator, which may be displayed in the UI.
  *
+ * To remove all accelerators for an action, use an empty, zero-terminated
+ * array for @accels.
+ *
  * Since: 3.12
  */
 void