Update file permissions.md

What does this MR do and why?

Provides clarification on a technicality regarding the user auth model. The current auth model prohibits Guests from viewing MRs as that permission is a user auth attribute. The attribute is overloaded whenever the project is set to public, effectively allowing for the guest user to view MR lists when it is typically prohibited by his role. Slack convo here.

I'm adding a 25th footnote to help clarify this as it currently reads that guests should not be able to view MRs. This isn't always true as anyone can visit https://gitlab.com/gitlab-org/gitlab/-/merge_requests unauthenticated to view MRs. This change accounts for that instance.

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Before	After

How to set up and validate locally

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.