[docs] [issue21366] Document that return in finally overwrites prev value

[Zachary Ware]

Changes by Zachary Ware <zachary.ware at gmail.com>:


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stage:  -> patch review
type: behavior -> enhancement
versions: +Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21366>
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