fixes #15311: mention in docs requirement for .btn-group-* sizing on immediate .btn-group

docs/_includes/components/button-groups.html

@@ -61,7 +61,7 @@
 {% endhighlight %}
 
   <h3 id="btn-groups-sizing">Sizing</h3>
-  <p>Instead of applying button sizing classes to every button in a group, just add <code>.btn-group-*</code> to the <code>.btn-group</code>.</p>
+  <p>Instead of applying button sizing classes to every button in a group, just add <code>.btn-group-*</code> each <code>.btn-group</code>, including when nesting multiple groups.</p>
   <div class="bs-example" data-example-id="button-group-sizing">
     <div class="btn-group btn-group-lg" role="group" aria-label="Large button group">
       <button type="button" class="btn btn-default">Left</button>