Move pkgbase changes to a new section at the start of the release
notes. This avoids conflating ports and pkgbase changes (which we
should avoid, since some users are already confused about the
relationship between pkgbase and ports), and provides important
information which might require user action earlier in the release
notes.
Be more explicit about when user action is and isn't required.
Remove "pkgbase" from user-facing documentation ("base system packages"
is the preferred user-facing terminology).
Suggest that people wanting to use installworld/installkernel should
depkgbasify, rather than suggesting DESTDIR=/. Overriding this check
is basically guaranteed to break the system, and should never be done;
it's only there in case of some sort of extraordinary situation where
the system can't be repaired any other way.