Merge branch 'docs-epic-issues-api-link' into 'master'

Epic issues API link

See merge request gitlab-org/gitlab-ee!3998

doc/api/epics.md

 # Epics API
 
-Every API call to epic_issues must be authenticated.
+Every API call to epic must be authenticated.
 
 If a user is not a member of a group and the group is private, a `GET` request on that group will result to a `404` status code.
 
 Epics are available only in EEU. If epics feature is not available a `403` status code will be returned.
 
+## Epic issues API
+
+The [epic issues API](epic_issues.md) allows you to interact with issues associated with an epic.
+
 ## List epics for a group
 
 Gets all epics of the requested group.
@@ -186,3 +190,4 @@ DELETE /groups/:id/-/epics/:epic_iid
 ```bash
 curl --header DELETE "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/groups/1/-/epics/5?title=New%20Title
 ```
+