Documentation improvements for Zabbix agent 2 as a Windows service

This was originally mentioned under ZBXNEXT-6023. Our documentation currently lacks the following information:

• Zabbix processes -> Agent 2, also applies to agents comparison chart:

  Agent 2 does not support daemonization.

  (after ZBXNEXT-6023, it's not fully correct - differences between platforms should be mentioned; for other Zabbix versions – for example, 5.0, the same situation).

• Installing agent 2 as a Windows service does not mention the information about versions of Agent 2 when this feature became available. 

Also, manpages for Zabbix agent and Zabbix agent 2 need to be updated to include --install and --uninstall options