[docs] [issue21514] update json module docs in light of RFC 7159 & ECMA-404

[Chris Rebert]

Chris Rebert added the comment:

Thanks for the speedy review!

Those NaN-related arguments are already mentioned in the docs (see last 2 sentences of https://docs.python.org/3/library/json.html#infinite-and-nan-number-values ), and this patch doesn't touch that subsection.

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