gh-101825: Clarify that as_integer_ratio() output is always normalized (#101843)

Make docstrings for `as_integer_ratio` consistent across types, and document that
the returned pair is always normalized (coprime integers, with positive denominator).

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Co-authored-by: Owain Davies <116417456+OTheDev@users.noreply.github.com>
Co-authored-by: Mark Dickinson <dickinsm@gmail.com>