[SPARK-13382][DOCS][PYSPARK] Update pyspark testing notes in build docs

## What changes were proposed in this pull request?

The current build documents don't specify that for PySpark tests we need to include Hive in the assembly otherwise the ORC tests fail.

## How was the this patch tested?

Manually built the docs locally. Ran the provided build command follow by the PySpark SQL tests.

![pyspark2](https://cloud.githubusercontent.com/assets/59893/13190008/8829cde4-d70f-11e5-8ff5-a88b7894d2ad.png)

Author: Holden Karau <holden@us.ibm.com>

Closes #11278 from holdenk/SPARK-13382-update-pyspark-testing-notes-r2.