Expand on privacy budgeting

[martinthomson]

Nit: this doc talks briefly about privacy budgeting in the privacy section. It also talks about trust and security in the MPC and TEE private computation options, but only in the context of keeping the data and computation private. Would it make sense to explicitly cover how to make privacy budgeting trusted?
Keeping un-replayable privacy budget state tends to require a trusted party (or multiple trusted parties who check on each other) as well as some sort of access control to prevent parties from consuming (or observing) each other's budgets.

Originally posted by @palenica in #14 (comment)

[eriktaubeneck]

Another comment referencing this: https://github.com/patcg/docs-and-reports/pull/14/files#r1001896672