Auto merge of #25742 - thombles:tk/StringCoercion, r=steveklabnik
Auto merge of #25742 - thombles:tk/StringCoercion, r=steveklabnik

A few of us [over on the forum](https://users.rust-lang.org/t/string-type-coercion-in-rust/1439) have been tripped up by this distinction, which I don't think is mentioned. It's kind of logical if you read the "Deref coercions" page and squint a bit but I think it would be nice to explain it directly. Here's one way we could clarify it.

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