Merge pull request #20279 from wagenet/update-features-md

Correct FEATURES.md

Author: chriskrycho

FEATURES.md

@@ -5,6 +5,12 @@ for a detailed explanation.
 
 ## Feature Flags
 
-* `ember-testing-resume-test`
+* `EMBER_DEFAULT_HELPER_MANAGER`
 
-  Introduces the `resumeTest` testing helper to complement the `pauseTest` helper.
+  Provides a default manager for unrecognized helpers as specified in
+  [RFC-756](https://github.com/emberjs/rfcs/blob/master/text/0756-helper-default-manager.md).
+
+* `EMBER_UNIQUE_ID_HELPER`
+
+  Provides a `{{unique-id}} helper as specified in
+  [RFC-659](https://github.com/emberjs/rfcs/blob/master/text/0659-unique-id-helper.md).