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

Changes by Serhiy Storchaka <storchaka+cpython@gmail.com>: ---------- keywords: +needs review stage: -> patch review status: pending -> open versions: +Python 3.5, Python 3.6, Python 3.7 -Python 3.4 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue1283110> _______________________________________