Add doc comment to hiding portions of code example

Refactor hiding example to be more complete

src/doc/rustdoc/src/documentation-tests.md

@@ -79,8 +79,9 @@ from your example, but are important to make the tests work. Consider
 an example block that looks like this:
 
 ```text
-/// Some documentation.
+/// /// Some documentation.
-# fn foo() {}
+/// # fn foo() {} // this function will be hidden
+/// println!("Hello, World!");
 ```
 
 It will render like this: