Slider patterns: Provide warning about inability to operate with touc…

…h-based assistive technologies due to lack of needed APIs (pull #1186)

Addresses issue #1150 for the slider patterns by adding a warning.
The warning explains that widgets using the pattern are not fully operable with touch-based assistive technologies
because the needed APIs do not yet exist.

Co-authored-by: Matt King <a11yThinker@Gmail.com>
Co-authored-by: Carolyn MacLeod <Carolyn_MacLeod@ca.ibm.com>

aria-practices.html

@@ -2344,7 +2344,10 @@ <h3>Slider</h3>
         A slider is an input where the user selects a value from within a given range.
         Sliders typically have a slider thumb that can be moved along a bar or track to change the value of the slider.
       </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>Examples</h4>
         <ul>
@@ -2407,6 +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>