docs: clarify how permissions are additive

[Galen Charlton]

Two subtle point that should be documented, perhaps in admin/permissions, are:

- If a user has a permission at more than one depth (because they have per-user permissions or are in secondary permissions groups), the depth that gives them the broadest access wins
- There is no way to override a permissions grant. I.e., if a permission is assigned via the user's profile, it cannot be revoked or restricted at the per-user level or via a secondary permissions group.

Evergreen master