Document `org-timestamp-up' and `org-timestamp-down' behavior

* org.el (org-timestamp-up): Document undocumented behavior.
(org-timestamp-up): The function changes the timestamp type when
the point is on the enclosing bracket.  The documentation now
reflects that behavior.

* org.el (org-timestamp-down): Document undocumented behavior.
(org-timestamp-down): The function changes the timestamp type when
the point is on the enclosing bracket.  The documentation now
reflects that behavior.

TINYCHANGE

lisp/org.el

@@ -17802,7 +17802,8 @@ NODEFAULT, 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,
-the day or the time, change that.
+the day or the time, change that.  If the cursor is on the enclosing
+bracket, change the timestamp type.
 With prefix ARG, change by that many units."
   (interactive "p")
   (org-timestamp-change (prefix-numeric-value arg) nil 'updown))
@@ -17810,7 +17811,8 @@ With prefix ARG, change by that many units."
 (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,
-the day or the time, change that.
+the day or the time, change that.  If the cursor is on the enclosing
+bracket, change the timestamp type.
 With prefix ARG, change by that many units."
   (interactive "p")
   (org-timestamp-change (- (prefix-numeric-value arg)) nil 'updown))