Mention the default location of alternative super block on a UFS2 file system, in fsck_ffs(8).
Mention the default location of alternative super block on a
UFS2 file system, in fsck_ffs(8).

Submitted by:	KOMATSU Shinichiro <koma2 at lovepeers ! org>
PR:		docs/86362
MFC After:	3 days

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