[docs] [issue38096] Clean up the "struct sequence" / "named tuple" docs

[Raymond Hettinger]

Change by Raymond Hettinger <raymond.hettinger at gmail.com>:


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pull_requests: +15636
pull_request: https://github.com/python/cpython/pull/16010

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38096>
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