Change wording for self-managed customers on top-level groups

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR changes the wording of the NOTE for self-managed users to use a top-level group, based on a discussing in this MR.

Although organizations with a self-managed instance currently cannot have a central overview of the entire organization on e.g. vulnerabilities, the choice for a single top-level group can also have other consequences and should therefore not be recommended without also discussing these consequences.

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