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