org.el (org-read-date): Point to Info documentation

* lisp/org.el (org-read-date): Add reference to Info documentation,
mainly because that describes all the available calendar navigation
commands.

lisp/org.el

@@ -13337,6 +13337,9 @@ The function understands only English month and weekday abbreviations.
 While prompting, a calendar is popped up - you can also select the
 date with the mouse (button 1).  The calendar shows a period of three
 months.  To scroll it to other months, use the keys `>' and `<'.
+There are many other calendar navigation commands available, see
+Info node `(org) The date/time prompt' for a full list.
+
 If you don't like the calendar, turn it off with
        (setq org-read-date-popup-calendar nil)