is_ascii on an empty string or slice returns true

[timvisee]

Update the description of the is_ascii functions - an empty string or slice also returns true.

This follows the pattern of all(). Clippy currently suggests to change string.chars().all(|c| c.is_ascii()) into string.is_ascii(). This suggestion therefore seems fitting.

I've already questioned the behavior for this multiple times. I've always had to check the internals to conclude how it works. That's why I'm opening this PR to add it directly in the documentation.

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[Noratrieb]

@bors r+ rollup

[bors]

📌 Commit 7a11c72 has been approved by Noratrieb

It is now in the queue for this repository.