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Rollup merge of #158244 - kantnero:attribute-docs-deprecated-warn, r=GuillaumeGomez,traviscross
Attribute docs `deprecated` , `warn`, `allow`, `cfg`, `deny`, and `forbid`
Added documentation for built-in `deprecated` , `warn`, `allow`, `cfg`, `deny`, and `forbid` attributes using the #[doc(attribute = "...")] mechanism.
Part of rust-lang/rust#157604
@rustbot r? @GuillaumeGomez
cc @fmease
cc @mejrs
