clearly document rules for snapshot revert/merge so it is clear to the role user

[trgill]

other roles that require a reboot for the changes to take effect. https://github.com/linux-system-roles/kernel_settings#kernel_settings_reboot_ok and https://github.com/linux-system-roles/kernel_settings#variables-exported-by-the-role

if the role needs to reboot for a change to take effect, if you set $ROLENAME_reboot_ok: true, then the role will reboot the machine. Otherwise, the role will set $ROLENAME_reboot_required to true to let the user know that a reboot is required for the changes to take effect.