nv.9: Editorial pass

Various rewording tweaks and clarifications to some behavior (e.g.
nvlist_size doesn't require calling nvlist_pack first).

Reviewed by: pjd
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D48256

(cherry picked from commit ae1cc4182b3fa6abe4de85ab201a198917dda73a)