We should document somewhere that we defensively clone the configuration when you initialize spark. This means users can't externally change the configuration after a SparkContext is created (which people might otherwise try to do because it appears mutable). It would be good to either explain this here or in the `SparkConf` class javadoc.

You could add something like this to the `SparkConf` javadoc:

    Once a SparkConf object is passed to Spark, it is defensively cloned and can no longer be modified by the user.