gh-158790: Address review comments

Use Sphinx roles in the NEWS entry (:meth:`list.insert`,
:keyword:`del`, :manpage:`memmove(3)`) and drop a comment in ins1()
that repeated the rationale already given in the commit message.

Author: hetaozdh

Misc/NEWS.d/next/Core_and_Builtins/2026-10-04-01-40-31.gh-issue-158790.F3kQ7z.rst

@@ -1 +1 @@
-Speed up ``list.insert()`` and ``del list[i]`` in free-threaded builds by using ``memmove()`` to shift elements when the list is not shared.
+Speed up :meth:`list.insert` and item deletion via :keyword:`del` in free-threaded builds by using :manpage:`memmove(3)`.

Objects/listobject.c

@@ -509,8 +509,6 @@ ins1(PyListObject *self, Py_ssize_t where, PyObject *v)
                                 n - where);
     }
 #else
-    /* The compiler expands this loop inline. This is faster than a
-       memmove() call for short lists. */
     for (Py_ssize_t i = n; --i >= where; )
         FT_ATOMIC_STORE_PTR_RELEASE(items[i+1], items[i]);
 #endif