Update EpicIssues OpenAPI documentation

What does this MR do and why?

Contributes to #377017

• Add fields description for EpicIssue entity
• Update EpicIssue API doc

Description texts are taken from GitLab REST API documentation.

Screenshots or screen recordings

Screenshots

For the purpose of showing it I mounted the EE::API::EpicIssues routes to lib/api/api.rb, but mounted routes are not included in this MR as EpicIssues are part of EE API (check the Note section on #377017). Few screenshots:

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How to set up and validate locally

1. Mount routes (add mount ::API::EpicIssues to lib/api/api.rb:184)
2. Run rake gitlab:openapi:generate
3. Review changes in doc/api/openapi/openapi_v2.yaml
4. Paste the content into https://editor.swagger.io/
5. Search for epic_issues references on page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.