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

[Inada Naoki]

Inada Naoki <songofacandy at gmail.com> added the comment:

OrderedDict is not recommended to preserve order any more.

Note that mention about `object_pairs_hook=OrderedDict` is removed intentionally.
https://github.com/python/cpython/pull/5001

Please forget about OrderedDict unless you need additional feature OrderedDict provides.  It is far inefficient than regular dict.

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nosy: +inada.naoki

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