docs: say that a scheduled job wakes the nook, and what it costs

[vish727]

cron was documented as "runs a command on a schedule" and nothing else, which left the question an author actually has unanswered: does it run when nobody is using the nook?

Until nookcloud/nookd#43 the honest answer was no — cron runs inside the machine, and the machine is stopped after ten idle minutes, so a nightly job on a nook nobody opens at 03:00 never fired. With that change it does, and this says so, along with what it costs in machine time: an entry every minute keeps the nook awake all the time; a nightly entry costs about ten minutes a night.

Worth being explicit about, because an author choosing * * * * * when 0 * * * * would do is now choosing something with a price.

Part of ENG-37