Update service accounts overview to include kibana-system

x-pack/docs/en/security/authentication/service-accounts.asciidoc

@@ -40,11 +40,15 @@ the format of `<namespace>/<service>`, where the `namespace` is a top-level
 grouping of service accounts, and `service` is the name of the service and
 must be unique within its namespace.
 
-Service accounts are predefined in code. Currently, only one service account is available:
+Service accounts are predefined in code. The following service accounts are
+available:
 
 `elastic/fleet-server`:: The service account used by the {fleet} server to
 communicate with {es}.
 
+`elastic/kibana-system`:: The service account used by {kib} to communicate with
+{es}.
+
 // tag::service-accounts-usage[]
 IMPORTANT: Do not attempt to use service accounts for authenticating individual
 users. Service accounts can only be authenticated with service tokens, which are