Suggest systemd override file instead of unit file for tmpdir (elasti…

…c#93211)

The systemd unit file is part of the Elasticsearch package and should
not be edited. Instead, we recommend creating a service override file.
This commit tweaks the docs for setting tmp dir with systemd to use the
override file instead of editing the unit file.

relates elastic#93121

docs/reference/setup/sysconfig/executable-jna-tmpdir.asciidoc

@@ -34,9 +34,8 @@ instance:
 export ES_TMPDIR=/usr/share/elasticsearch/tmp
 --------------------------------------------
 
-* If you are using `systemd` to run {es} as a service, using the `systemctl`
-command, add the following line to the `[Service]` section of your
-`elasticsearch.service` unit file:
+* If you are using `systemd` to run {es} as a service, add the following
+line to the `[Service]` section in a <<systemd,service override file>>:
 +
 [source,text]
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