Mention that making list comprehensions sugar for genexps will remove exposure

of the loop variable.

pep-3000.txt

@@ -67,7 +67,8 @@ Core language
 * ``True`` and ``False`` become keywords [4]_
 * ``as`` becomes a keyword [5]_
 * Have list comprehensions be syntactic sugar for passing an
-  equivalent generator expression to ``list()``.
+  equivalent generator expression to ``list()``; as a consequence the
+  loop variable will no longer be exposed.
 * Comparisons other than ``==`` and ``!=`` between disparate types
   will raise an exception unless explicitly supported by the type