Minor fixes in API doc

doc/api/README.md

@@ -42,7 +42,7 @@ following locations:
 - [Group milestones](group_milestones.md)
 - [Namespaces](namespaces.md)
 - [Notes](notes.md) (comments)
-- [Threaded comments](discussions.md)
+- [Discussions](discussions.md) (threaded comments)
 - [Notification settings](notification_settings.md)
 - [Open source license templates](templates/licenses.md)
 - [Pages Domains](pages_domains.md)

doc/api/notes.md

 # Notes API
 
-Notes are comments on snippets, issues or merge requests.
+Notes are comments on snippets, issues, merge requests or epics.
 
 ## Issues