Replace doc_auto_cfg with doc_cfg feature.

Author: Sinan Nalkaya

mbedtls/src/lib.rs

@@ -9,7 +9,7 @@
 #![deny(warnings)]
 #![allow(unused_doc_comments, ambiguous_glob_reexports)] // allow ambiguous glob reexports for now in autogenerated bindings.
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(nightly, feature(doc_auto_cfg))]
+#![cfg_attr(nightly, feature(doc_cfg))]
 
 #[cfg(not(any(feature = "std", feature = "no_std_deps")))]
 compile_error!("Either the `std` or `no_std_deps` feature needs to be enabled");