Improve description in group audit events UI

What does this MR do?

The Group Audit Events feature only displays group-level events, not project-level events within the groupw, which is unexpected behavior for many users. This improves the description that appears at the top of the feature page to better explain that only group-level events will be displayed.

No changelog because this is effectively a docs-only change.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #218782 (closed)