Add doc for epic state change notifications

doc/user/discussions/index.md

@@ -5,6 +5,7 @@ The ability to contribute conversationally is offered throughout GitLab.
 You can leave a comment in the following places:
 
 - issues
+- epics
 - merge requests
 - snippets
 - commits

doc/workflow/notifications.md

@@ -71,18 +71,18 @@ Below is the table of events users can be notified of:
 | Group access level changed   | User                                                              | Sent when user group access level is changed |
 | Project moved                | Project members [1]                                               | [1] not disabled |
 
-### Issue / Merge request events
+### Issue / Epics / Merge request events
 
 In most of the below cases, the notification will be sent to:
 
 - Participants:
   - the author and assignee of the issue/merge request
   - authors of comments on the issue/merge request
-  - anyone mentioned by `@username` in the issue/merge request title or description
-  - anyone mentioned by `@username` in any of the comments on the issue/merge request
+  - anyone mentioned by `@username` in the issue/epic/merge request title or description
+  - anyone mentioned by `@username` in any of the comments on the issue/epic/merge request
      ...with notification level "Participating" or higher
 - Watchers: users with notification level "Watch"
-- Subscribers: anyone who manually subscribed to the issue/merge request
+- Subscribers: anyone who manually subscribed to the issue/epic/merge request
 - Custom: Users with notification level "custom" who turned on notifications for any of the events present in the table below
 
 | Event                  | Sent to |
@@ -106,6 +106,8 @@ In most of the below cases, the notification will be sent to:
 | Failed pipeline        | The author of the pipeline |
 | Successful pipeline    | The author of the pipeline, if they have the custom notification setting for successful pipelines set |
 | New epic               | |
+| Close epic             | |
+| Reopen epic            | |
 
 In addition, if the title or description of an Issue or Merge Request is
 changed, notifications will be sent to any **new** mentions by `@username` as