This specifies the URL (not just hostname) of a proxy web server
through which to pass page fetch requests. Only host and (optional)
port are used from the given URL. If empty, no proxy is used: pages
are fetched directly.
Note that Proxy Auto-Config URL overrides this.
In Windows Control Panel, the Internet Options →
Connections tab → LAN settings →
Use a proxy server for your LAN host/port value is equivalent to
the Search Appliance's Proxy setting.