LUDOC-390 misc: Document shutdown procedure

LUDOC-390 misc: Document shutdown procedure

This patch creates a short section to document how to stop
the entire Lustre filesystem.  Currently the manual only details
the startup procedure and how to shut down a specific target
by using umount /mnt/mnt_point.

Signed-off-by: Joseph Gmitter <joseph.gmitter@intel.com>
Change-Id: I1fd9123177433a539cab72f1edc165bcdf3e39f4
Reviewed-on: https://review.whamcloud.com/28902
Tested-by: Jenkins
Reviewed-by: Saurabh Tandan <saurabh.tandan@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>