[SPARK-4437] update doc for WholeCombineFileRecordReader

update doc for WholeCombineFileRecordReader

Author: Davies Liu <davies@databricks.com>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #3301 from davies/fix_doc and squashes the following commits:

1d7422f [Davies Liu] Merge pull request #2 from JoshRosen/whole-text-file-cleanup
dc3d21a [Josh Rosen] More genericization in ConfigurableCombineFileRecordReader.
95d13eb [Davies Liu] address comment
bf800b9 [Davies Liu] update doc for WholeCombineFileRecordReader
[SPARK-4855][mllib] testing the Chi-squared hypothesis test

This PR tests the pyspark Chi-squared hypothesis test from this commit: c8abddc5164d8cf11cdede6ab3d5d1ea08028708 and moves some of the error messaging in to python.

It is a port of the Scala tests here: [HypothesisTestSuite.scala](https://github.com/apache/spark/blob/master/mllib/src/test/scala/org/apache/spark/mllib/stat/HypothesisTestSuite.scala)

Hopefully, SPARK-2980 can be closed.

Author: jbencook <jbenjamincook@gmail.com>

Closes #3679 from jbencook/master and squashes the following commits:

44078e0 [jbencook] checking that bad input throws the correct exceptions
f12ee10 [jbencook] removing checks for ValueError since input tests are on the Scala side
7536cf1 [jbencook] removing python checks for invalid input
a17ee84 [jbencook] [SPARK-2980][mllib] adding unit tests for the pyspark chi-squared test
3aeb0d9 [jbencook] [SPARK-2980][mllib] bringing Chi-squared error messages to the python side