docs: clarify geoip service health checks uses lua records

docs: clarify geoip service health checks uses lua records

The geoip backend documentation mentions:

> Additionally, the GeoIP backend can be used to provide service over
> several clusters, any of which can be taken out of use easily, for
> example for maintenance purposes.

But no details on how that is achieved. Without knowing Lua records
exist, it's hard to find how to do it.

Add a reference to the Lua records documentation to make it easier to
see the connection.

Signed-off-by: Kevin Daudt <me@ikke.info>