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

[R. David Murray]

R. David Murray added the comment:

Thanks!

While this patch does sort-of go into a detail, it seems to me like it does it in a tutorial-appropriate fashion.  I'm +1 on applying this.

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stage: needs patch -> commit review

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