org.el (org-read-date-display-live): Enhance docstring

* lisp/org.el (org-read-date-display-live): Document that live
display is only available when `org-read-date-popup-calendar' is
non-nil.

See <https://orgmod.org/list/20200630180259.zj3krk57x5iqmbsj@orion>

Reported-by: Edmund Christian Herenz <eherenz@eso.org>

lisp/org.el

@@ -2657,7 +2657,9 @@ stamps outside of this range."
 
 (defcustom org-read-date-display-live t
   "Non-nil means display current interpretation of date prompt live.
-This display will be in an overlay, in the minibuffer."
+This display will be in an overlay, in the minibuffer.  Note that
+live display is only active when `org-read-date-popup-calendar'
+is non-nil."
   :group 'org-time
   :type 'boolean)