[SPARK-11335][STREAMING] update kafka direct python docs on how to get the offset ranges for a KafkaRDD

tdas koeninger

This updates the Spark Streaming + Kafka Integration Guide doc with a working method to access the offsets of a `KafkaRDD` through Python.

Author: Nick Evans <me@nicolasevans.org>

Closes #9289 from manygrams/update_kafka_direct_python_docs.