Adds documentation for buttons in link groups

Mostly a duplication of the *Linked items* section, but for buttons.
Includes a note not to use the standard btn classes.

docs/_includes/components/list-group.html

@@ -74,6 +74,27 @@
   <a href="#" class="list-group-item">Porta ac consectetur ac</a>
   <a href="#" class="list-group-item">Vestibulum at eros</a>
 </div>
+{% endhighlight %}
+
+  <h2 id="list-group-buttons">Button items</h2>
+  <p>List group items may be buttons instead of list items (that also means a parent <code>&lt;div&gt;</code> instead of an <code>&lt;ul&gt;</code>). No need for individual parents around each element. <strong class="text-danger">Don't use the standard <code>.btn</code> classes here.</strong></p>
+  <div class="bs-example" data-example-id="list-group-btns">
+    <div class="list-group">
+      <button class="list-group-item">Cras justo odio</button>
+      <button class="list-group-item">Dapibus ac facilisis in</button>
+      <button class="list-group-item">Morbi leo risus</button>
+      <button class="list-group-item">Porta ac consectetur ac</button>
+      <button class="list-group-item">Vestibulum at eros</button>
+    </div>
+  </div>
+{% highlight html %}
+<div class="list-group">
+  <button class="list-group-item">Cras justo odio</button>
+  <button class="list-group-item">Dapibus ac facilisis in</button>
+  <button class="list-group-item">Morbi leo risus</button>
+  <button class="list-group-item">Porta ac consectetur ac</button>
+  <button class="list-group-item">Vestibulum at eros</button>
+</div>
 {% endhighlight %}
 
   <h2 id="list-group-disabled">Disabled items</h2>

docs/_includes/nav/components.html

@@ -124,6 +124,7 @@
     <li><a href="#list-group-basic">Basic example</a></li>
     <li><a href="#list-group-badges">Badges</a></li>
     <li><a href="#list-group-linked">Linked items</a></li>
+    <li><a href="#list-group-buttons">Button items</a></li>
     <li><a href="#list-group-disabled">Disabled items</a></li>
     <li><a href="#list-group-contextual-classes">Contextual classes</a></li>
     <li><a href="#list-group-custom-content">Custom content</a></li>