*: Use auto_doc_cfg instead of doc(cfg) attributes (#2983)

Co-authored-by: João Oliveira <hello@jxs.pt>
Co-authored-by: Thomas Eizinger <thomas@eizinger.io>

protocols/dcutr/src/lib.rs

@@ -21,6 +21,8 @@
 //! Implementation of the [libp2p Direct Connection Upgrade through Relay
 //! specification](https://github.com/libp2p/specs/blob/master/relay/DCUtR.md).
 
+#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
+
 pub mod behaviour;
 mod handler;
 mod protocol;