[docs] [issue30550] Document order-preserving dictionary output in json

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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pull_requests: +15110
pull_request: https://github.com/python/cpython/pull/15403

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue30550>
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