bpo-30940: specify what happens if ndigits is None or omitted

[elenaoat]

I've added the latest changes that were mentioned in the bug tracker.

https://bugs.python.org/issue30940

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[rhettinger]

Rather than lose information with this edit, I would prefer to make it more precise, "For a general Python object, round(number) or round(number, None) delegates to number.__round__(), and round(number, ndigits) delegates to number.__round__(ndigits)."

[elenaoat]

I've modified the text accordingly.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I didn't expect the Spanish Inquisition!. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[ned-deily]

Based on more recent discussion on the tracker, it looks these proposed changes have now been addressed by other PRs, primarily #6342 and #2740. Thanks for your contribution @elenaoat!