Docs: &A and &mut A are references, not pointers

[SimonSapin]

Caught by Brian Smith: #27774 (comment)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[sanxiyn]

@bors r+

[bors]

📌 Commit 761d163 has been approved by sanxiyn

[steveklabnik]

@bors: rollup