Document that form field feedback icons only work with `input.form-control`

Closes #14202.

[skip sauce]

docs/_includes/css/forms.html

@@ -588,6 +588,7 @@
 
   <h3>With optional icons</h3>
   <p>You can also add optional feedback icons with the addition of <code>.has-feedback</code> and the right icon.</p>
+  <p><strong class="text-danger">Feedback icons only work with textual <code>&lt;input class="form-control"&gt;</code> elements.</strong></p>
   <div class="bs-callout bs-callout-warning">
     <h4>Icons, labels, and input groups</h4>
     <p>Manual positioning of feedback icons is required for inputs without a label and for <a href="../components#input-groups">input groups</a> with an add-on on the right. You are strongly encouraged to provide labels for all inputs for accessibility reasons. If you wish to prevent labels from being displayed, hide them with the <code>sr-only</code> class. If you must do without labels, adjust the <code>top</code> value of the feedback icon. For input groups, adjust the <code>right</code> value to an appropriate pixel value depending on the width of your addon.</p>