Documentation/dax: Update Usage section

Update the Usage section to reflect the new individual dax selection
functionality.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>