org-agenda: Clarify docstring

* lisp/org-agenda.el (org-scheduled-past-days): Clarify docstring.

lisp/org-agenda.el

@@ -1284,9 +1284,9 @@ shown, either today or the nearest into the future."
 
 (defcustom org-scheduled-past-days 10000
   "Number of days to continue listing scheduled items not marked DONE.
-When an item is scheduled on a date, it shows up in the agenda on this
-day and will be listed until it is marked done for the number of days
-given here."
+When an item is scheduled on a date, it shows up in the agenda on
+this day and will be listed until it is marked done or for the
+number of days given here."
   :group 'org-agenda-daily/weekly
   :type 'integer)