Issue #14205: document the change of dict[key] behaviour if dict is modified

during the lookup

Doc/library/stdtypes.rst

@@ -2208,6 +2208,10 @@ pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
       See :class:`collections.Counter` for a complete implementation including
       other methods helpful for accumulating and managing tallies.
 
+      .. versionchanged:: 3.3
+         If the dict is modified during the lookup, a :exc:`RuntimeError`
+         exception is now raised.
+
    .. describe:: d[key] = value
 
       Set ``d[key]`` to *value*.