Add link to rust-lang/rust#67295

serde/src/private/mod.rs

@@ -1,7 +1,9 @@
 pub mod de;
-pub mod doc;
 pub mod ser;
 
+// FIXME: #[cfg(doctest)] once https://github.com/rust-lang/rust/issues/67295 is fixed.
+pub mod doc;
+
 pub use lib::clone::Clone;
 pub use lib::convert::{From, Into};
 pub use lib::default::Default;