insights: indicate datapoint limit is just for backfilled data, so users know they'll get snapshots

Created by: Joelkw

The text currently says:

Show the past 1 year and 10 months of data, one datapoint every 2 months. This insight provides 12 datapoints.

We should make it:

Backfill the past 1 year and 10 months of data, one datapoint every 2 months. This insight provides 12 backfilled datapoints, and displays a daily-refreshed snapshot that will save future points at monthly intervals.

Except I need to wordsmith that still (ideas are welcome), just jotting this down now.