Make the doc_auto_cfg a crate-level attribute.

Co-authored-by: John Nunley <jtnunley01@gmail.com>

src/lib.rs

@@ -37,7 +37,8 @@
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
 )]
-#[cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+
 use std::fmt;
 use std::marker::PhantomData;
 use std::panic::{RefUnwindSafe, UnwindSafe};