Issue #14205: Document the dict lookup change in What's New in Python 3.3

Doc/whatsnew/3.3.rst

@@ -474,6 +474,13 @@ Some smaller changes made to the core Python language are:
 
   (:issue:`12170`)
 
+* A dict lookup now raises a :exc:`RuntimeError` if the dict is modified during
+  the lookup. If you implement your own comparaison function for objects used
+  as dict keys and the dict is shared by multiple threads, access to the dict
+  should be protected by a lock.
+
+  (:issue:`14205`)
+
 
 New and Improved Modules
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