docs: difference between `@stdlib/regexp/whitespace` and `\s`?

### Description

The [doc of regex/whitespace](https://stdlib.io/docs/api/latest/@stdlib/regexp/whitespace) says:

> Returns a [regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions) to match a [white space](https://en.wikipedia.org/wiki/Whitespace_character) character.

But [\s does the same thing](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_expressions/Character_classes):

> Matches a single white space character, including space, tab, form feed, line feed, and other Unicode spaces. Equivalent to [\f\n\r\t\v\u0020\u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff]. For example, /\s\w*/ matches " bar" in "foo bar".

I think an explanation in the doc can be helpful. The git history of the library also doesn't shed any light on the reason. (Perhaps it was created at a time when JS was less powerful?)

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### Demo

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### Reproduction

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### Expected Results

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### Actual Results

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### Version

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### Environments

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### Checklist

- [x] Read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [x] Searched for existing issues and pull requests.

Start with the linked @stdlib/regexp/whitespace API documentation and compare its stated behavior with JavaScript's \s character class. Update the documentation to explain the distinction or equivalence, with the work complete when readers can understand why this utility exists; the payload names no source file or test.

索引モデルが issue の本文から書いたものです。