gh-129813, PEP 782: Add doc reference link (#138986)

Author: vstinner

Doc/c-api/bytes.rst

@@ -228,6 +228,7 @@ called with a non-bytes parameter.
       The function is :term:`soft deprecated`,
       use the :c:type:`PyBytesWriter` API instead.
 
+.. _pybyteswriter:
 
 PyBytesWriter
 -------------

Doc/whatsnew/3.15.rst

@@ -718,7 +718,8 @@ New features
   and :c:data:`Py_mod_abi`.
   (Contributed by Petr Viktorin in :gh:`137210`.)
 
-* Implement :pep:`782`, the :c:type:`PyBytesWriter` API. Add functions:
+* Implement :pep:`782`, the :ref:`PyBytesWriter API <pybyteswriter>`.
+  Add functions:
 
   * :c:func:`PyBytesWriter_Create`
   * :c:func:`PyBytesWriter_Discard`