ox: Display a visible help for scrolling dispatcher's buffer

* lisp/ox.el (org-export--dispatch-ui): Display a help message in
  header line for scrolling dispatcher's buffer

lisp/ox.el

@@ -5478,7 +5478,8 @@ back to standard interface."
 	(delete-other-windows)
 	(org-switch-to-buffer-other-window
 	 (get-buffer-create "*Org Export Dispatcher*"))
-	(setq cursor-type nil)
+	(setq cursor-type nil
+	      header-line-format "Use SPC, DEL, C-n or C-p to navigate.")
 	;; Prevent square brackets from being highlighted when point
 	;; moves onto them.
 	(modify-syntax-entry ?\[ "w"))