*: 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>

core/src/lib.rs

@@ -35,6 +35,8 @@
 //!   define how to upgrade each individual substream to use a protocol.
 //!   See the `upgrade` module.
 
+#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
+
 #[allow(clippy::derive_partial_eq_without_eq)]
 mod keys_proto {
     include!(concat!(env!("OUT_DIR"), "/keys_proto.rs"));