Merge branch '10795-add-epic-tree-BE-initial-support-docs' into 'master'

Update Epic Link documentation

See merge request gitlab-org/gitlab-ce!28684

doc/api/epic_links.md

@@ -125,7 +125,7 @@ POST /groups/:id/epics/:epic_iid/epics
 | --------------- | -------------- | -------- | ------------------------------------------------------------------------------------------------------------------ |
 | `id`            | integer/string | yes      | The ID or [URL-encoded path of the group](README.md#namespaced-path-encoding) owned by the authenticated user      |
 | `epic_iid`      | integer        | yes      | The internal ID of the (future parent) epic.                                                                                       |
-| `title` | integer        | yes      | The global ID of the child epic. Internal ID can't be used because they can conflict with epics from other groups. |
+| `title`         | string         | yes      | The title of a newly created epic. |
 
 ```bash
 curl --header POST "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/groups/1/epics/5/epics?title=Newpic
@@ -141,6 +141,7 @@ Example response:
   "group_id": 49,
   "parent_id": 23,
   "has_children": false,
+  "has_issues": false,
   "reference":  "&2",
   "url": "http://localhost/groups/group16/-/epics/2",
   "relation_url": "http://localhost/groups/group16/-/epics/1/links/24"