Add group specific email headers documentation

Add X-GitLab-Group-Id and X-GitLab-Group_Path documentation

doc/user/profile/notifications.md

@@ -203,16 +203,18 @@ Notification email messages include GitLab-specific headers. You can filter the
 The following table lists all GitLab-specific email headers:
 
 | Header                      | Description                                                             |
-|-----------------------------|-------------------------------------------------------------------------|
-| X-GitLab-Project            | The name of the project the notification belongs to                     |
-| X-GitLab-Project-Id         | The ID of the project                                                   |
-| X-GitLab-Project-Path       | The path of the project                                                 |
-| X-GitLab-(Resource)-ID      | The ID of the resource the notification is for, where resource is `Issue`, `MergeRequest`, `Commit`, etc|
-| X-GitLab-Discussion-ID      | Only in comment emails, the ID of the thread the comment is from    |
-| X-GitLab-Pipeline-Id        | Only in pipeline emails, the ID of the pipeline the notification is for |
-| X-GitLab-Reply-Key          | A unique token to support reply by email                                |
-| X-GitLab-NotificationReason | The reason for being notified: one of `mentioned`, `assigned`, or `own_activity` |
-| List-Id                     | The path of the project in a RFC 2919 mailing list identifier useful for email organization, for example, with Gmail filters |
+|------------------------------------|-------------------------------------------------------------------------|
+| X-GitLab-Group-Id **(PREMIUM)**   | The group's ID. Only present on notification emails for epics.         |
+| X-GitLab-Group-Path **(PREMIUM)** | The group's path. Only present on notification emails for epics.       |
+| X-GitLab-Project                   | The name of the project the notification belongs to.                     |
+| X-GitLab-Project-Id                | The project's ID.                                                   |
+| X-GitLab-Project-Path              | The project's path.                                                 |
+| X-GitLab-(Resource)-ID             | The ID of the resource the notification is for. The resource, for example, can be `Issue`, `MergeRequest`, `Commit`, or another such resource. |
+| X-GitLab-Discussion-ID             | The ID of the thread the comment belongs to, in notification emails for comments.    |
+| X-GitLab-Pipeline-Id               | The ID of the pipeline the notification is for, in notification emails for pipelines. |
+| X-GitLab-Reply-Key                 | A unique token to support reply by email.                                |
+| X-GitLab-NotificationReason        | The reason for the notification. This can be `mentioned`, `assigned`, or `own_activity`. |
+| List-Id                            | The path of the project in an RFC 2919 mailing list identifier. This is useful for email organization with filters, for example. |
 
 ### X-GitLab-NotificationReason