guide: use "C" in extern items

nrc · nrc · commit e1588d5375da · 2017-09-18T19:50:42.000+12:00
diff --git a/guide/items.md b/guide/items.md
@@ -110,3 +110,10 @@ Use `{}` for the full definition of the macro.
 macro_rules! foo {
 }
 ```
+
+
+### 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" { ... }.
