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Auto merge of #114780 - RalfJung:io-safety, r=Amanieu
add more explicit I/O safety documentation
Fixes https://github.com/rust-lang/unsafe-code-guidelines/issues/434
Cc https://github.com/rust-lang/rust/issues/114167
Cc `@Manishearth` `@sunfishcode` `@joshtriplett`
