Document recommendation about Arc::clone(&ref) syntax.

[nical]

See rust-lang/rust#63252 and rust-lang/rfcs#1954.

[dtolnay]

This is a useful recommendation but I don't think it is a great fit for the API guidelines. I would prefer to keep this project focused on public API design, and leave implementation-level tips for a different document. That way we keep the list of guidelines short enough that it's reasonable to run through the whole thing when polishing a crate.