The nextboot(8) manual page currently says that the loader(8) would delete

The nextboot(8) manual page currently says 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
for both Forth and Lua implementations: they would not delete it, but just
change the setting to "NO", that is, disable it.

While at it, add one missing serial (Oxford) comma and fix some bogus line
wraps along the way.

Approved by:	bcr (manpages)
Differential Revision:	https://reviews.freebsd.org/D25971