Document what the icons mean in the query log

The following should be documented both in the app and somewhere in the docs:

• Database/server icon: cache is enabled and the DNS response came from cache
• Flag: The response came from the DNS rules OR the upstream DNS server replied with 0.0.0.0 or ::1
• Left pointing arrow: The answer was forwarded to a DNS server
• Questionmark: Unknown what happened with the query (normally you shouldn't see that)