Clarify use of serverURLroot

[planetf1]

At https://odpi.github.io/egeria-docs/guides/admin/servers/configuring-a-metadata-access-point/#set-the-server-url-root we explain serverURL root

I have had a few people discuss this on slack & get confused when running in a container or complex/vpn environment

We need to clarify that this address is used for other servers in the cohort to call this server's REST endpoints - or at least that it is the default for that. Effectively it is how this cohort member advertizes its services to other members