zabbix-agent: clarify python-zabbix-api is needed locally

roles/zabbix-agent/tasks/main.yml

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-# If you do not have python-zabbix-api installed, this will fail. Please
-# install it manually. It is needed for the tasks later in this role
+# If you do not have python-zabbix-api installed (** ON YOUR LOCAL MACHINE **),
+# this will fail. Please install it; it is needed for tasks later in this role.
 - name: install python-zabbix-api package on localhost
   local_action: pacman name=python-zabbix-api
   run_once: true