[docs] [issue27000] improve document of filter

[Xiang Zhang]

Xiang Zhang added the comment:

First I have to clarify that my mistake is not in understanding but in writing. What I mean by 'identify the return value True or False' is actually what you say, 'evaluate for truth or falsehood'. I also notice the lowercase false and true in the doc. I know they are deliberate. Sorry about this.

For ``bool``, I almost agree with you now. Although I still think it's telling readers incorrect info in the second part. For ``bool``, it is not equivalent to ``(item for item in iterable if function(item))`` but ``(item for item in iterable if item)``. For CPython, you are not telling the truth.

And for identity function, I insist. I don't see any advantage with this sentence other than confusion. I don't think this will affect other implementation either.

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<http://bugs.python.org/issue27000>
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