fix #9454: add grid docs about responsive column clearing

_includes/nav-css.html

@@ -20,6 +20,7 @@
     <li><a href="#grid-example-basic">Ex: Stacked-to-horizonal</a></li>
     <li><a href="#grid-example-mixed">Ex: Mobile-desktop</a></li>
     <li><a href="#grid-example-mixed-complete">Ex: Mobile, tablet, desktop</a></li>
+    <li><a href="#grid-responsive-clearing">Responsive column clearing</a></li>
     <li><a href="#grid-offsetting">Offset columns</a></li>
     <li><a href="#grid-nesting">Nested columns</a></li>
     <li><a href="#grid-column-ordering">Column ordering</a></li>

css.html

@@ -326,6 +326,21 @@ base_url: "../"
       <p><a class="btn btn-danger" target="_blank" href="http://examples.getbootstrap.com/grid/">More grid examples</a></p>
     </div>
 
+    <h3 id="grid-responsive-clearing">Responsive column clearing</h3>
+    <p>With the four tiers of grids available you're bound to run into issues where, at certain breakpoints, your columns don't clear quite right as one is taller than the other. To fix that, use a combination of a <code>.clearfix</code> and our <a href="../css#responsive-utilities">responsive utility classes</a>.</p>
+{% highlight html %}
+<div class="row">
+  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
+  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
+
+  <!-- Add the extra clearfix for only the required viewport -->
+  <div class="clearfix visible-xs"></div>
+
+  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
+  <div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
+</div>
+{% endhighlight %}
+
     <h3 id="grid-offsetting">Offsetting columns</h3>
     <p>Move columns to the right using <code>.col-md-offset-*</code> classes. These classes increase the left margin of a column by <code>*</code> columns. For example, <code>.col-md-offset-4</code> moves <code>.col-md-4</code> over four columns.</p>
     <div class="bs-docs-grid">