DOC: afsd man page: give an example of the direct volume mount syntax

The syntax is a little confusing, so an example is needed to clarify it.

Change-Id: I413a5f2af6ccf48e780007c658c35a34384d09e0
Reviewed-on: http://gerrit.openafs.org/10281
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>