Update access_and_permissions.md with IP restriction

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/443400 where we identified that comments on issues or merge requests made via email replies cannot be gated with IP restrictions, as the user's true IP may not be accessible, and imposing this may in turn be a breaking change. IP access restrictions aren't intended to be a firewall, and this is now documented in the docs for that feature.

See https://gitlab.com/gitlab-org/gitlab/-/issues/443400#note_1790548274 for additional discussion.

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