Help: Add versionadded for string(TIMESTAMP) %f specifier

This was accidentally left out of commit c050d6a01e (string(TIMESTAMP):
add %f specifier for microseconds, 2022-01-27, v3.23.0-rc1~59^2).

Help/command/string.rst

@@ -491,6 +491,8 @@ specifiers:
   The second of the current minute.  60 represents a leap second. (00-60)
 
 ``%f``
+  .. versionadded:: 3.23
+
   The microsecond of the current second (000000-999999).
 
 ``%U``