[docs] [issue1283110] Give __len__() advice for "don't know"

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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keywords: +needs review
stage:  -> patch review
status: pending -> open
versions: +Python 3.5, Python 3.6, Python 3.7 -Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue1283110>
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