[SPARK-19516][DOC] update public doc to use SparkSession instead of SparkContext

## What changes were proposed in this pull request?

After Spark 2.0, `SparkSession` becomes the new entry point for Spark applications. We should update the public documents to reflect this.

## How was this patch tested?

N/A

Author: Wenchen Fan <[email protected]>

Closes #16856 from cloud-fan/doc.

Author: cloud-fan