guide: use "C" in extern items

nrc · nrc · commit 4164de43fc3f · 2017-11-13T17:11:44.000+13:00
diff --git a/guide/items.md b/guide/items.md
@@ -256,3 +256,9 @@ where
     body
 }
 ```
+
+### extern items
+
+When writing extern items (such as `extern "C" fn`), always be explicit about
+the ABI. For example, write `extern "C" fn foo ...`, not `extern fn foo ...`, or
+`extern "C" { ... }.
