[docs] [issue27000] improve document of filter

[Xiang Zhang]

Xiang Zhang added the comment:

Not about code, just the doc. In my opinion, if ``bool`` is not called it is definitely not equivalent to ``(item for item in iterable if function(item))``, which actually calls the function, even there is nothing different in the result. But, this is a rather subjective and not important now. I am OK with all your opinions. And considering other interpreters, leaving it untouched is a good idea.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27000>
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