Document that .progress-bar-striped doesn't work in IE<=9; fixes #16058

[skip sauce]

docs/_includes/components/progress-bars.html

@@ -116,7 +116,7 @@
 {% endhighlight %}
 
   <h2 id="progress-striped">Striped</h2>
-  <p>Uses a gradient to create a striped effect. Not available in IE8.</p>
+  <p>Uses a gradient to create a striped effect. Not available in IE9 and below.</p>
   <div class="bs-example" data-example-id="striped-progress-bar">
     <div class="progress">
       <div class="progress-bar progress-bar-success progress-bar-striped" role="progressbar" aria-valuenow="40" aria-valuemin="0" aria-valuemax="100" style="width: 40%">