fixes #15340: mention in docs that most selects can't change border-radius

docs/_includes/css/forms.html

@@ -378,6 +378,7 @@
 {% endhighlight %}
 
   <h3>Selects</h3>
+  <p>Note that many native select menus—namely in Safari and Chrome—have rounded corners that cannot be modified via <code>border-radius</code> properties.</p>
   <div class="bs-example" data-example-ids="select-form-control">
     <form>
       <select class="form-control">