Add a description of soft updates journaling to tunefs(8).

Requested by: Graham Perrin
PR: 261944
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 82493ff7007abb5a2d059c74748d2907dafc5395)
(cherry picked from commit de1f80e4c1386614a8b6c7b31d861d20930995cc)