sbin/{ffsinfo,mount,newfs}: reference ffs(4) in man pages

With some copy-editing to account for the man page being in
section 7 on stable/14.

PR: 282867
MFC: stable/14
Approved by: mhorne (via IRC)

(cherry picked from commit 3a212cc66ad1b5d6c1dc97eb07143e919a123491)