Add note about --fs-group option

Author: Oleg Bulatov

install_config/registry/deploy_registry_existing_clusters.adoc

@@ -79,6 +79,10 @@ To see a full list of options that you can specify when creating the registry:
 ----
 $ oc adm registry --help
 ----
+
+Please note that the value for `--fs-group` must be
+xref:../../install_config/persistent_storage/pod_security_context.adoc#fsgroup[permitted by the SCC]
+used by the registry (typically, the restricted SCC).
 endif::[]
 
 [[registry-daemonset]]