Document how cron with proc works

[Earlopain]

Functionality for this was added somewhat recently but from the example in the readme it's not clear to me how that actually works. I guess my questions would be:

• How frequently is the block called? Every second seems likely
• What do I need to return to make the job be scheduled? It looks like I a time in the past may cause the job to be scheduled

It would be nice if docs can be expanded for this.