document system indices api privilege (#12391)

* update privilege requirement

docs/static/security/logstash.asciidoc

@@ -123,16 +123,9 @@ privileges  for the Logstash indices. You can create roles from the
 ---------------------------------------------------------------
 POST _security/role/logstash_reader
 {
-  "indices": [
-    {
-      "names": [ "logstash-*" ], <1>
-      "privileges": ["read","view_index_metadata"]
-    }
-  ]
+  "cluster": ["manage_logstash_pipelines"]
 }
 ---------------------------------------------------------------
-<1> If you use a custom Logstash index pattern, specify that pattern
-instead of the default `logstash-*` pattern.
 
 . Assign your Logstash users the `logstash_reader` role. If the Logstash user
 will be using
@@ -149,7 +142,7 @@ POST _security/user/logstash_user
   "full_name" : "Kibana User for Logstash"
 }
 ---------------------------------------------------------------
-<1> `logstash_admin` is a built-in role that provides access to `.logstash-*`
+<1> `logstash_admin` is a built-in role that provides access to system
 indices for managing configurations.
 
 [float]