Note the change of name in FreeBSD 5.0.

PR:		212542
Approved by:	bcr (mentor)
MFC after:	4 days
Differential Revision:	https://reviews.freebsd.org/D8105

sbin/mount_unionfs/mount_unionfs.8

@@ -31,7 +31,7 @@
 .\"	@(#)mount_union.8	8.6 (Berkeley) 3/27/94
 .\" $FreeBSD$
 .\"
-.Dd November 30, 2006
+.Dd October 3, 2016
 .Dt MOUNT_UNIONFS 8
 .Os
 .Sh NAME
@@ -328,9 +328,13 @@ accessible via
 .Xr mount_nullfs 8
 .Sh HISTORY
 The
-.Nm
+.Nm mount_null
 utility first appeared in
 .Bx 4.4 .
+It was renamed to
+.Nm
+in
+.Fx 5.0 .
 .Pp
 The
 .Fl r