Clarify that number of threads is set by packages

This commit clarifies that on systemd the packages already set the number of
threads for the Elasticsearch process.

Relates #27840

docs/reference/setup/sysconfig/threads.asciidoc

@@ -2,7 +2,7 @@
 === Number of threads
 
 Elasticsearch uses a number of thread pools for different types of operations.
-It is important that it is able to create new threads whenever needed.   Make
+It is important that it is able to create new threads whenever needed. Make
 sure that the number of threads that the Elasticsearch user can create is at
 least 4096.
 
@@ -10,3 +10,6 @@ This can be done by setting <<ulimit,`ulimit -u 4096`>> as root before
 starting Elasticsearch, or by setting `nproc` to `4096` in
 <<limits.conf,`/etc/security/limits.conf`>>.
 
+The package distributions when run as services under `systemd` will configure
+the number of threads for the Elasticsearch process automatically. No
+additional configuration is required.