gh-133968: Update PyUnicodeWriter_WriteASCII() documentation (#135297)

The function was added to Python 3.14.

Doc/c-api/unicode.rst

@@ -1827,7 +1827,7 @@ object.
    On success, return ``0``.
    On error, set an exception, leave the writer unchanged, and return ``-1``.
 
-   .. versionadded:: next
+   .. versionadded:: 3.14
 
 .. c:function:: int PyUnicodeWriter_WriteWideChar(PyUnicodeWriter *writer, const wchar_t *str, Py_ssize_t size)
 

Doc/whatsnew/3.15.rst

@@ -298,12 +298,6 @@ New features
   functions as replacements for :c:func:`PySys_GetObject`.
   (Contributed by Serhiy Storchaka in :gh:`108512`.)
 
-* Add :c:func:`PyUnicodeWriter_WriteASCII` function to write an ASCII string
-  into a :c:type:`PyUnicodeWriter`. The function is faster than
-  :c:func:`PyUnicodeWriter_WriteUTF8`, but has an undefined behavior if the
-  input string contains non-ASCII characters.
-  (Contributed by Victor Stinner in :gh:`133968`.)
-
 * Add :c:type:`PyUnstable_Unicode_GET_CACHED_HASH` to get the cached hash of
   a string. See the documentation for caveats.
   (Contributed by Petr Viktorin in :gh:`131510`)