new line at end of nac_docs; update to entry 4

docs/_includes/nav_docs.html

@@ -39,4 +39,4 @@
       </ul>
     </div>
   {% endfor %}
-</div>
+</div>

docs/cookbook/04-self-closing-tag.md

@@ -9,4 +9,6 @@ next: jsx-root-node-count.html
 
 In JSX, `<MyComponent />` alone is valid while `<MyComponent>` isn't.
 
-Related: every React component can be self-closing: `<div/>`.
+> Note:
+>
+> Every React component can be self-closing: `<div />`. `<div></div>` is also an equivalent.