docs: adding specific instructions to create operator namespace

When a user attempts to adhere to the current documentation to use alternative
namespaces for both the operator and the cluster, they fail because
our common YAML file only has a single namespace for the cluster. This
change adds specific instructions to create operator namespace.

Closes: #13079

Signed-off-by: Redouane Kachach <rkachach@redhat.com>

Documentation/Storage-Configuration/Advanced/ceph-configuration.md

@@ -23,6 +23,14 @@ to also change `ROOK_OPERATOR_NAMESPACE` to create a new Rook Operator for each
 forget to set `ROOK_CURRENT_NAMESPACE_ONLY`), or you can leave it at the same value for every
 Ceph cluster if you only wish to have one Operator manage all Ceph clusters.
 
+If the operator namespace is different from the cluster namespace, the operator namespace must be
+created before running the steps below. (The cluster namespace does not need to be created first,
+as it will be created by `common.yaml` in the script below.)
+
+```console
+kubectl create namespace $ROOK_OPERATOR_NAMESPACE
+```
+
 This will help you manage namespaces more easily, but you should still make sure the resources are
 configured to your liking.