Update docs to describe philosophy of dealing with slack when using the framework

[philipkent]

1. Framework does not call self.core_reset or self.core.break_realtime(), or delay() for you. It is therefore up to you to account for lost slack due to overhead from processing on the core device for the framework.
2. Standard method is to call self.core.reset() in initialize_devices() and then self.core.break_realtime() in before_scan_point()