chore(destructors): add clarification to code (#416)

Co-authored-by: Marco Ieni <[email protected]>

src/idioms/dtor-finally.md

@@ -9,6 +9,10 @@ be used to run code that must be run before exit.
 ## Example
 
 ```rust,ignore
+fn baz() -> Result<(), ()> {
+    // some code
+}
+
 fn bar() -> Result<(), ()> {
     // These don't need to be defined inside the function.
     struct Foo;