Add doc to Cassandra processors to clarify which fields are ignored if using a controller service

As of NIFI-5621, the Cassandra processors can use a controller service to provide cassandra sessions. The controller service takes a number of properties that are also specified in the processors. If the controller service is used, the similar properties are ignored, but there is no documentation as such, which can lead to confusion and errors. For example, if you specify the keyspace name in the processor properties but not in the controller service properties, the processor is invalid.

The documentation should accurately reflect which properties are being used in which scenarios.