jolynyao

Update the [flex event proposal](https://github.com/WICG/attribution-reporting-api/blob/main/flexible_event_config.md) to clarify which features are available for testing and which are not (summary buckets & multiple trigger specs)

Add some reference to [event_level_epsilon](https://github.com/WICG/attribution-reporting-api/pull/1097) to the main explainer, likely the [Data limits & noise section](https://github.com/WICG/attribution-reporting-api/blob/main/EVENT.md#data-limits-and-noise). We may also want to add a reference to the flex event explainer and/or...