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

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

(cherry picked from commit 0929a153fc5cfe35f7390ab5870a36daed19b0ce)
(cherry picked from commit 759ee8654012dc35380eed6204bf4f35f6eacaaf)