test for missing_doc in the external_doc test

Author: QuietMisdreavus

src/test/rustdoc/auxiliary/external-cross.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(external_doc)]
+#![deny(missing_doc)]
 
 #[doc(include="external-cross-doc.md")]
 pub struct NeedMoreDocs;