update json syntax section in ARM

https://jenkins.isc.org/job/Kea_doc/KeaAdministratorReferenceManual/index.html#json-syntax

The configuration file consists of a single object (often colloquially called a map) started with a curly bracket. It comprises one or more of the “Dhcp4”, “Dhcp6”, “DhcpDdns”, “Control-agent”, and “Netconf” objects. It is possible to define additional elements but they will be ignored.

and

The specification of several supported elements (e.g. “Dhcp4”, “Dhcp6”) in a single configuration file can be confusing and works badly with the commands that fetch and write new configurations. Support for it will be removed in a future release of Kea, after which each component will require its own configuration file.

since 1.7.10 we do not support this.