docs snapshot for rook version master

docs/rook/master/ceph-cluster-crd.md

@@ -319,6 +319,8 @@ A Placement configuration is specified (according to the kubernetes PodSpec) as:
 The `mon` pod does not allow `Pod` affinity or anti-affinity. Instead, `mon`s have built-in anti-affinity with each other through the operator. The operator determines which nodes should run a `mon`. Each `mon` is then tied to a node with a node selector using a hostname.
 See the [mon design doc](https://github.com/rook/rook/blob/master/design/ceph/mon-health.md) for more details on the `mon` failover design.
 
+If you use `labelSelector` for `osd` pods, you must write two rules both for `rook-ceph-osd` and `rook-ceph-osd-prepare` like [the example configuration](https://github.com/rook/rook/blob/master/cluster/examples/kubernetes/ceph/cluster-on-pvc.yaml#L68). It comes from the design that there are these two pods for an OSD. For more detail, see the [osd design doc](https://github.com/rook/rook/blob/master/design/ceph/dedicated-osd-pod.md) and [the related issue](https://github.com/rook/rook/issues/4582).
+
 The Rook Ceph operator creates a Job called `rook-ceph-detect-version` to detect the full Ceph version used by the given `cephVersion.image`. The placement from the `mon` section is used for the Job.
 
 ### Cluster-wide Resources Configuration Settings