Make argument clinic summary entry parallel to other PEP entries.

The existing entry repeated the same info that was in the body text, which
means it didn't satisfy the sections "brevity" requirement :)

Doc/whatsnew/3.4.rst

@@ -126,12 +126,10 @@ CPython implementation improvements:
 * :pep:`442`: :ref:`Safe object finalization <whatsnew-pep-442>`
 * :pep:`445`: :ref:`Configurable memory allocators <whatsnew-pep-445>`
 * :pep:`456`: :ref:`Secure and interchangeable hash algorithm <whatsnew-pep-456>`
+* :pep:`436`: :ref:`Argument Clinic <whatsnew-pep-436>`.
 * Improve finalization of Python modules to avoid setting their globals
   to None, in most cases (:issue:`18214`).
 * A more efficient :mod:`marshal` format (:issue:`16475`).
-* "Argument Clinic", an initial step towards providing improved introspection
-  support for builtin and standard library extension types implemented in C
-  (:pep:`436`)
 
 Please read on for a comprehensive list of user-facing changes.
 
@@ -976,6 +974,8 @@ Changes to Python's build process and to the C API include:
   marked as accepting ``const char *`` rather than ``char *`` (Contributed
   by Serhiy Storchaka in :issue:`1772673`).
 
+.. _whatsnew-pep-436:
+
 * "Argument Clinic" (:pep:`436`) is now part of the CPython build process
   and can be used to simplify the process of defining and maintaining
   accurate signatures for builtins and standard library extension modules