gh-37733: Bell polynomials univariate variable fix, complete and ordinary variants
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I've changed the implementation of `bell_polynomial` so that
`bell_polynomial(n,n) == x0^n` instead of `x^n` because when `k < n`,
`bell_polynomial(n,k)` is a polynomial in variables `x0, ..., xn-k`.
I added a relevant example to illustrate the new behaviour (there were
none for the old behaviour) and tests for the degenerate examples.
This is based upon my suggestion proposed in issue #37727.
After [suggestions](https://github.com/sagemath/sage/issues/37727#issuec
omment-2036002403) from @tscrim, I changed `k` to be an optional
argument with default value `None`. If `k is None`, complete Bell
polynomials is returned instead. Also, support for ordinary analogues
was added. Documentation, examples and tests were subsequently updated.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/37733
Reported by: Nolord
Reviewer(s): Nolord, Travis Scrimshaw
