[docs] [issue28393] Update encoding lookup docs wrt #27938

[Marc-Andre Lemburg]

Marc-Andre Lemburg added the comment:

The names are indeed case-insensitive.

However, something important is missing: the implementation details changed between Python 3.5 and 3.6. Please check the implementation for details and update the documentation with versionadded flags. Thanks.

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nosy: +lemburg

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