[issue21046] Document formulas used in statistics

[Ezio Melotti]

Ezio Melotti added the comment:

> E.g.
> median([x1, x2, x3, x4, x5]) = x3
> median([x1, x2, x3, x4, x5, x6]) = (x3 + x4) / 2

The docs seem to already contain similar examples for some of the functions (e.g. median()), but not for others (e.g. mean()).
For these, if the formula can be expressed with a simple Python equivalent (e.g. sum(values) / len(values)), I think it would be reasonable to add it.

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nosy: +ezio.melotti

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