Update Epic Link documentation

What does this MR do?

Backport of the Epic Links documentation change from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13246

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content