add reference link to heartbeat div, see phetsims/joist#140

js/accessibility/AccessibleSiblingStyle.js

@@ -73,8 +73,9 @@ define( require => {
       // 'fixed' positioned elements interfere with workarounds that are meant to prevent Safari from going to sleep
       // when the browser is left inactive for a few minutes. This z-index keeps the PDOM from interfering, while still
       // allowing us to use `fixed`. If the PDOM elements are ever styled with position: 'absolute' (would require
-      // changing the current approach of AccessiblePeer.positionElements), this could be removed.
-      'z-index: -1;' +
+      // PDOM elements to be positioned relative to focusable ancestors rather than viewport), this could be removed.
+      // See https://github.com/phetsims/joist/blob/master/js/Heartbeat.js as well for the workaround.
+      'z-index: 1;' +
 
       // JUST FOR DEBUGGING! So you can see the PDOM on top of other graphical content
       // 'z-index: 5000;' +