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

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. ---------- stage: needs patch -> commit review _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21739> _______________________________________