org.el: org-timestamp-up/down: mention time changes.

* org.el (org-timestamp-up, org-timestamp-down): mention time
changes in the docstring.

lisp/org.el

@@ -15456,16 +15456,16 @@ hour and minute fields will be nil if not given."
 
 (defun org-timestamp-up (&optional arg)
   "Increase the date item at the cursor by one.
-If the cursor is on the year, change the year.  If it is on the month or
-the day, change that.
+If the cursor is on the year, change the year.  If it is on the month,
+the day or the time, change that.
 With prefix ARG, change by that many units."
   (interactive "p")
   (org-timestamp-change (prefix-numeric-value arg) nil 'updown))
 
 (defun org-timestamp-down (&optional arg)
   "Decrease the date item at the cursor by one.
-If the cursor is on the year, change the year.  If it is on the month or
-the day, change that.
+If the cursor is on the year, change the year.  If it is on the month,
+the day or the time, change that.
 With prefix ARG, change by that many units."
   (interactive "p")
   (org-timestamp-change (- (prefix-numeric-value arg)) nil 'updown))