relnotes: clarify that snapshots were previously possible without SU+J

A journalist reviewing FreeBSD 13.2 interpreted "it is now possible to
take snapshots on UFS filesystems when running with journaled soft
updates" to mean that UFS snapshots were a new feature in 13.2 and
required SU+J to be enabled. Clarify that snapshots were already
available, but not compatible with SU+J.

Reviewed by: gjb (earlier), karels
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39630