Bug 1565515 - Added note declaring that key list must be kept sorted r=asuth,ttung

Differential Revision: https://phabricator.services.mozilla.com/D37855

--HG--
extra : moz-landing-system : lando

dom/indexedDB/test/unit/test_keys.js

@@ -32,6 +32,7 @@ function* testSteps() {
   let store = db.createObjectStore("store");
 
   // Test simple inserts
+  // Note: the keys must be in order
   var keys = [
     -1 / 0,
     -1.7e308,