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

Changes by Raymond Hettinger <raymond.hettinger@gmail.com>: ---------- assignee: rhettinger -> r.david.murray _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21739> _______________________________________