Document justified navbar for #15906

Document in the source code that the justified nav is meant for single line list items only. This should help prevent Issues like #15906 in the future.

docs/examples/justified-nav/index.html

@@ -32,6 +32,8 @@
 
     <div class="container">
 
+      <!-- The justified navigation menu is meant for single line per list item.
+           Multiple lines will require custom code not provided by Bootstrap. -->
       <div class="masthead">
         <h3 class="text-muted">Project name</h3>
         <nav>