HeaderBar: Explain use in conjunction w/ GtkWindow

After hinting how good GtkHeaderBar is for GtkWindow, let’s link to
the latter and indicate how users can make the two work together.

gtk/gtkheaderbar.c

@@ -51,6 +51,10 @@
  *
  * GtkHeaderBar can add typical window frame controls, such as minimize,
  * maximize and close buttons, or the window icon.
+ *
+ * For these reasons, GtkHeaderBar is the natural choice for use as the custom
+ * titlebar widget of a #GtkWindow (see gtk_window_set_titlebar()), as it gives
+ * features typical of titlebars while allowing the addition of child widgets.
  */
 
 #define DEFAULT_SPACING 6