Replace 'positive integer' with 'nonnegative integer'

[blkerby]

This PR replaces all instances of "positive integer" with "nonnegative integer" in the configuration docs, as in every case zero is also an accepted value, and in many cases zero is already singled out in the docs as a possibility.

A "max_width" of zero of course does not give reasonable results (and it results in "left behind trailing whitespace" errors), but a value of 1 is just as bad, so there seems to be no harm in replacing "positive integer" with "nonnegative integer" even in this case.

Fixes #6570.

[ytmimi]

Thanks for the PR! I'd like to replace positive integer with whole number to better reflect that zero is a valid option. Feel free to suggest changes to the wording, but I'd like to avoid using the term nonnegative.

[ytmimi]

nonnegative is technically correct, but there's something about the double negative that I don't like. The set of Whole numbers also includes zero and positive integer values. How about this?

- **Possible values**: Any whole number in the range [0, [`max_width`](#max_width)]

[blkerby]

A drawback is that the term whole number has three different common definitions, making it less clear; e.g., see https://mathworld.wolfram.com/WholeNumber.html. The double negative in "nonnegative integer" is annoying but I don't know of any other unambiguous alternative. From what I've seen, "nonnegative integer" is the more-or-less standard terminology in math and computer science, with exceptions like in elementary school where "whole number" is used (as the concept of negative numbers isn't yet introduced), or in type theory or fundamentals of mathematics where "natural number" is commonly used (e.g. coming logically before constructing the set of integers), though that term is also ambiguous since some authors exclude zero from "natural numbers".