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

[ziaee]

Summary

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 <[e-mail]>

[mhorne]

Approved.

[editor_callfortesting.org]

Perhaps note 'exit' to continue with regular boot?

[ziaee]

In D49988#1140553, @editor_callfortesting.org wrote:

Perhaps note 'exit' to continue with regular boot?

The single user interface documents that already, so I think it isn't necessary. We could document that, but this isn't an appropriate place for it and I don't know where it would be.