Add warning to multi-thumb slider pattern

aria-practices.html

@@ -2410,6 +2410,10 @@ <h3>Slider (Multi-Thumb)</h3>
         Conversely, the minimum value of the upper end thumb is limited by the current value of the lower end thumb.
         However, in some multi-thumb sliders, each thumb sets a value that does not depend on the other thumb values.
       </p>
+      <p class="warning">
+        Users of touch-based assistive technologies may not be able to fully operate widgets that implement this pattern because APIs for capturing the necessary gestures and commands from assistive technologies are not yet available.
+        Authors should fully test widgets using this pattern with assistive technologies before considering incorporation into production systems.
+      </p>
 
       <section class="notoc">
         <h4>Example</h4>