[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[Raymond Hettinger]

Changes by Raymond Hettinger <raymond.hettinger at gmail.com>:


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assignee: rhettinger -> r.david.murray

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21739>
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