Attribute docs deny and forbid

[kantnero]

Added documentation for built-in deny and forbid attributes using the #[doc(attribute = "...")] mechanism.

Part of #157604

@rustbot r? @GuillaumeGomez
cc @fmease
cc @mejrs

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[rustbot]

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

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