More precisely define the contribution budget and window in the spec

[alexmturner]

The spec currently leaves the budget as fully implementation-defined (see here). The explainer, however, describes Chrome's implementation plan.

While it may be difficult to align on exact details across implementations, we should probably define the privacy unit more precisely while leaving some key parameters as implementation-defined (e.g. the precise epsilon/budget and perhaps the timespan used in the privacy unit).