Add clarification about three empty spans for the navbar-burger

It was not clear in the documentation that you *need* to have the 3 empty spans or otherwise the lines (and the cross) wouldn't show up.

docs/documentation/components/navbar.html

@@ -634,7 +634,7 @@ $(document).ready(function() {
 
 <div class="content">
   <p>
-    The <code>navbar-burger</code> is a hamburger menu that only appears on <strong>mobile</strong>. It has to appear as the last child of <code>navbar-brand</code>.
+    The <code>navbar-burger</code> is a hamburger menu that only appears on <strong>mobile</strong>. It has to appear as the last child of <code>navbar-brand</code>. It has to contain three empty <code>span</code> tags in order to visualize the hamburger lines or the cross (when active).
   </p>
 </div>