[docs] [issue27000] improve document of filter

[Raymond Hettinger]

Raymond Hettinger added the comment:

> bool is not enough of a special case to call it out without 
> confusing the issue.

I concur.  It would be easy to make the docs less usable by elaborating on this special case.  For the most part, a user should use None if they just want to test the truth value of the input.

The docs for filter() have been through a number of revisions and much discussion.  Let's not undo previous efforts.  For the most part, these docs have been successful in communicating what filter() does.

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nosy: +rhettinger

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