Update `std::io::Error::downcast` return type and update its doc according to decision made by rust libs-api team in https://github.com/rust-lang/rust/issues/99262#issuecomment-1894246216
Update `std::io::Error::downcast` return type and update its doc according to decision made by rust libs-api team in https://github.com/rust-lang/rust/issues/99262#issuecomment-1894246216