EE port: Add help texts to K8 form fields

What does this MR do?

EE-port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27274

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