doc/ceph-volume systemd describe the interaction with 'simple' activation

Signed-off-by: Alfredo Deza <adeza@redhat.com>

diff --git a/doc/ceph-volume/simple/systemd.rst b/doc/ceph-volume/simple/systemd.rst
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+.. _ceph-volume-simple-systemd:
+
+systemd
+=======
+Upon startup, it will identify the logical volume by loading the JSON file in
+``/etc/ceph/osd/{id}-{uuid}.json`` corresponding to the instance name of the
+systemd unit.
+
+After identifying the correct volume it will then proceed to mount it by using
+the OSD destination conventions, that is::
+
+    /var/lib/ceph/osd/{cluster name}-{osd id}
+
+For our example OSD with an id of ``0``, that means the identified device will
+be mounted at::
+
+
+    /var/lib/ceph/osd/ceph-0
+
+
+Once that process is complete, a call will be made to start the OSD::
+
+    systemctl start ceph-osd@0
+
+The systemd portion of this process is handled by the ``ceph-volume simple
+trigger`` sub-command, which is only in charge of parsing metadata coming from
+systemd and startup, and then dispatching to ``ceph-volume simple activate`` which
+would proceed with activation.