User's guide update: JSON section needs a refresh

The section 5.1.2: JSON syntax (https://oldkea.isc.org/docs/kea-guide.html#json-format) mentions putting all configuration in one file. This is no longer recommended way, as it causes issues with config-write. The text should be rephrased to say that entries other than Dhcp4, Dhcp6 may be present (ca, d2, netconf).