Clarify number of columns in nested grid

css.html

@@ -403,7 +403,7 @@ base_url: "../"
 
 
     <h3 id="grid-nesting">Nesting columns</h3>
-    <p>To nest your content with the default grid, add a new <code>.row</code> and set of <code>.col-md-*</code> columns within an existing <code>.col-md-*</code> column. Nested rows should include a set of columns that add up to 12.</p>
+    <p>To nest your content with the default grid, add a new <code>.row</code> and set of <code>.col-md-*</code> columns within an existing <code>.col-md-*</code> column. Nested rows should include a set of columns that add up to 12 or less.</p>
     <div class="row show-grid">
       <div class="col-md-9">
         Level 1: .col-md-9