Right now, the nextboot(8) manual page suggests that the loader(8) would delete the /boot/nextboot.conf file or its contents which is 1) not the most user-friendly way of working with custom configurations, and 2) simply not true.
In reality, what happens is that when next-boot kernel is being configured, nextboot_enable="YES" line is added at the top of that file. After loader(8) reads the configuration, it would not delete it, but simply change the setting to "NO", that is, disable it.
While here, add one missing serial (Oxford) comma and fix some bogus line wraps along the way.