Clarify scenario where AsciiExt appears to operate on non-ASCII

[frewsxcv]

Fixes #31203

[rust-highfive]

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[frewsxcv]

r? @steveklabnik

[steveklabnik]

Showing the imports is considered better nowadays.

[steveklabnik]

Thanks so much! A few nits, and also, could you wrap this to 80 cols, not 100? libstd uses 80.

[frewsxcv]

Comments have been addressed

[steveklabnik]

@bors: r+ rollup

thanks so much

[bors]

📌 Commit 93d6425 has been approved by steveklabnik