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
@@ -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.
