bpo-33234: Add exact allocation optimization to lists in What's New (GH-10200)

In commit 372d705d a new optimization to the list() type was introduced but it was not added
to the optimization section in What's new for Python 3.8.

Doc/whatsnew/3.8.rst

@@ -242,6 +242,11 @@ Optimizations
   Note that this means that instances can no longer be weak-referenced and
   that arbitrary attributes can no longer be added to them.
 
+* The :class:`list` constructor does not overallocate the internal item buffer
+  if the input iterable has a known length (the input implements ``__len__``).
+  This makes the created list 12% smaller on average. (Contributed by Pablo
+  Galindo in :issue:`33234`.)
+
 
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