docs: mark enterprise runner visibility required

[gustavlasko]

If the visibility property is not specified on the github_enterprise_actions_runner_group, an error is thrown.

Pull request checklist

• [X] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Nope!

Please see our docs on breaking changes to help!

• [ ] Yes
• [X] No

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