Mention TCP keepalive configuration in the radioroom installation guide

[envirover]

UV RradioRoom relies on TCP keepalive mechanism to detect broken connections. Reliable detection of broken connection is required to reconnect to restarted UV Hub server and to failover to ISBD channel when TCP channel is not healthy. The 7200 seconds system default setting for keepalive time is typically too long. It's recommended to adjust the system default settings for tcp_keepalive_time, tcp_keepalive_intvl, and tcp_keepalive_probes. See https://tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepalive.html for TCP keepalive configuration HOWTOs.