[DOC] Document that roles with DLS enabled should not have write access

[davidlago]

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Coming from this issue, we need to add a note in the DLS/FLS documentation explicitly calling out that when roles have DLS rules enabled, they should not have write access as users can "write themselves out of the rules" (i.e. changing the values of fields used for filtering documents out)

[davidlago]

Oops! It seems like the issue I link had already been moved to this repo. Closing this one.