Clarify which options map to Kafka settings, and which are native to this plugin

[robbavey]

The current documentation states:

Some of these options map to a Kafka option. Defaults usually reflect the Kafka default setting, and might change if Kafka’s consumer defaults change.

This leaves the burden of understanding on the user of the plugin, to be able to know which of these settings are set directly on the consumer/producer, and which of them are not. We might also want to surface more clearly which defaults are supplied by us, and which are supplied by the kafka libraries we include in the plugin