Clarify not all approvals available at group level

What does this MR do and why?

This MR updates the merge request approvals docs to clarify that not all MR approval options available at the project level are available at the group level.

This is to help deflect tickets like the one that brought this confusion to light. GitLab team members with access to Zendesk can learn more in the ticket. This MR addresses the #383024 issue, which is freely available.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.