persistentLogging out-of-date documentation

[majorz]

We state here that:

"To enable persistent logs in a running device, add “persistentLogging”: true to /mnt/boot/config.json and reboot."

This is no longer true as persistent logging should be enabled from the dashboard or the API. If that is done manually on a live device, supervisor will revert to disabled persistent logging once the device is rebooted.

Modifying persistentLogging in config.json only can be done before first boot of the device in which case supervisor will enable it on the dashboard as well.