Add hint about related setting for REX concurrency

Fixes #38247 (Missing docs on how to REX batching works)

Author: maximiliankolb

guides/common/modules/ref_advanced-settings-in-the-job-wizard.adoc

@@ -43,6 +43,9 @@ Is used if effective user is different from the ssh user.
 Concurrency level::
 Defines the maximum number of jobs executed at once.
 This can prevent overload of system resources in a case of executing the job on a large number of hosts.
++
+If `foreman_proxy_ansible_batch_size` is set, {SmartProxies} run batches of concurrent remote execution jobs.
+You can change this setting in the {ProjectWebUI} under *Administer* > *Settings* on the *Ansible* tab.
 
 Execution ordering::
 Determines the order in which the job is executed on hosts.