Revert "Revert doc cfg from Eq impls due to rustdoc bug"

Fixed by rust-lang/rust#79300.

This reverts commit d9fb416.

Author: dtolnay

codegen/src/eq.rs

@@ -113,8 +113,7 @@ fn expand_impl(defs: &Definitions, node: &Node) -> TokenStream {
 
     let eq = quote! {
         #cfg_features
-        // https://github.com/rust-lang/rust/issues/79279
-        // #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
+        #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
         impl Eq for #ident {}
     };