A note that build.warnings and CARGO_BUILD_WARNINGS don't apply to MIRI

[peter-lyons-kehl]

Thanks for the pull request 🎉!
Please read the contribution guide: https://doc.crates.io/contrib/.

What does this PR try to resolve?

rust-lang/miri#5202

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @epage (or someone else) some time within the next two weeks.

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[epage]

Does this belong in Cargo's docs or in miris'?

cargo miri is an unstable command and I would consider this a bug on their side (even if they consider it wontfix)