mount.8: Add a single example for single user mode

The zfs command to do this is a bit longer and harder to remember. In
the last few releases mount(8) learned how to do this, so lets show it
in the manual.

MFC after:		3 days
Reported by:		Jan Bramkamp <[email protected]>
Discussed with:		cperciva, emaste
Reviewed by:		mhorne
Approved by:		mhorne (mentor)
Differential Revision:	https://reviews.freebsd.org/D49988

(cherry picked from commit c3e06b23b4)

sbin/mount/mount.8

@@ -547,6 +547,10 @@ for more information.)
 .It Pa /etc/fstab
 file system table
 .El
+.Sh EXAMPLES
+Remount the root filesystem with read-write permissions:
+.Pp
+.Dl mount -uw /
 .Sh DIAGNOSTICS
 Various, most of them are self-explanatory.
 .Pp