doc: add section on direct volume access

doc: add section on direct volume access

Provide examples of the direct volume access syntax, using the
fictitious example.com cell.

Reviewed-on: http://gerrit.openafs.org/6691
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
(cherry picked from commit e99224f2fe049bc339e87c8b6c195de67dca2f08)

Change-Id: I5b2ac3b6f255d5918eeea4a63d4c7bb6164961d5
Reviewed-on: http://gerrit.openafs.org/6755
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>