[docs] [issue40976] Clarify motivation for `chain.from_iterable`

New submission from Raymond Hettinger <raymond.hettinger@gmail.com>: Thank you for the suggestion, but we're going to decline for the reasons mentioned in the PR. Feel free to submit other ideas. ---------- nosy: +rhettinger resolution: -> rejected stage: patch review -> resolved status: open -> closed _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue40976> _______________________________________