Add comment to warn about incorrect scope usage in project policy

What does this MR do and why?

This change adds a comment to explain why a condition with an incorrect scope is being kept in place temporarily so that it doesn't get removed again and cause another incident. Functionality currently depends on this incorrect scope, so it cannot be removed right now even though it's a bug.

References

!210067 (comment 2843326807) https://gitlab.com/gitlab-org/gitlab/-/issues/578561#note_2868029408

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