bsdinstall: Improve auto-partition message

Manually tuning ZFS for systems with <8GB ram hasn't been necessary at
least since the switch to OpenZFS. We have users reporting using 1GB RAM
with no manual tuning/issues. Further, the page this links to is a stale
wiki page, which is causing complaints. Remove this misleading note and
replace it with a similar message for UFS. While here, reword that note
to be a bit clearer.

PR: 287719
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D50971

(cherry picked from commit 5ed26c21e4ff1d478d4611abbf3dc14cc1b77244)