doc: Added PG states.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>

diff --git a/doc/cluster-ops/pg-states.rst b/doc/cluster-ops/pg-states.rst
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+========================
+ Placement Group States
+========================
+
+When checking a cluster's status (e.g., running ``ceph -w`` or ``ceph -s``), 
+Ceph will report on the status of the placement groups. A placement group has 
+one or more states. The optimum state for placement groups in the placement group
+map is ``active + clean``. 
+
+*Creating*
+  Ceph is still creating the placement group.
+
+*Active*
+  Ceph will process requests to the placement group.
+
+*Clean*
+  Ceph replicated all objects in the placement group the correct number of times.
+
+*Down*
+  A replica with necessary data is down, so the placement group is offline.
+
+*Replay*
+  The placement group is waiting for clients to replay operations after an OSD crashed.
+
+*Splitting*
+  Ceph is splitting the placment group into multiple placement groups. (functional?)
+
+*Scrubbing*
+  Ceph is checking the placement group for inconsistencies.
+
+*Degraded*
+  Ceph has not replicated some objects in the placement group the correct number of times yet.
+
+*Inconsistent*
+  Ceph detects inconsistencies in the one or more replicas of an object in the placement group
+  (e.g. objects are the wrong size, objects are missing from one replica *after* recovery finished, etc.).
+
+*Peering*
+  The placement group is undergoing the peering process
+
+*Repair*
+  Ceph is checking the placement group and repairing any inconsistencies it finds (if possible).
+
+*Recovering*
+  Ceph is migrating/synchronizing objects and their replicas.
+
+*Backfill*
+  Ceph is scanning and synchronizing the entire contents of a placement group
+  instead of inferring what contents need to be synchronized from the logs of
+  recent operations. *Backfill* is a special case of recovery.
+
+*Incomplete*
+  Ceph detects that a placement group is missing a necessary period of history 
+  from its log.  If you see this state, report a bug, and try to start any
+  failed OSDs that may contain the needed information.
+
+*Stale*
+  The placement group is in an unknown state - the monitors have not received
+  an update for it since the placement group mapping changed.
+
+*Remapped*
+  The placement group is temporarily mapped to a different set of OSDs from what
+  CRUSH specified.