Document that $(...).button('reset') is async.

Closes #16362.
[skip sauce]

docs/_includes/js/buttons.html

@@ -120,7 +120,7 @@
   <p>Toggles push state. Gives the button the appearance that it has been activated.</p>
 
   <h4><code>$().button('reset')</code></h4>
-  <p>Resets button state - swaps text to original text.</p>
+  <p>Resets button state - swaps text to original text. <strong>This method is asynchronous and returns before the resetting has actually completed.</strong></p>
 
   <h4><code>$().button(string)</code></h4>
   <p>Swaps text to any data defined text state.</p>