Clarify 'Threshold days' description for scrub task

When configuring a Scrub schedule, any changes to the Threshold settings aren't reflected in the calendar.

For example, if I set a Scrub to run every Monday but set a Threshold to 13 or 14, so that the Scrub runs every other week as I'd like it to (every 14 days), the calendar still shows Scurbs scheduled for every Monday rather than every other Monday.