WeichenXu123 and srowen [SPARK-23254][ML] Add user guide entry and example for DataFrame mult…
…ivariate summary

## What changes were proposed in this pull request?

Add user guide and scala/java/python examples for `ml.stat.Summarizer`

## How was this patch tested?

Doc generated snapshot:

![image](https://user-images.githubusercontent.com/19235986/38987108-45646044-4401-11e8-9ba8-ae94ba96cbf9.png)
![image](https://user-images.githubusercontent.com/19235986/38987096-36dcc73c-4401-11e8-87f9-5b91e7f9e27b.png)
![image](https://user-images.githubusercontent.com/19235986/38987088-2d1c1eaa-4401-11e8-80b5-8c40d529a120.png)
![image](https://user-images.githubusercontent.com/19235986/38987077-22ce8be0-4401-11e8-8199-c3a4d8d23201.png)

Author: WeichenXu <weichen.xu@databricks.com>

Closes #20446 from WeichenXu123/summ_guide.
Latest commit 59c3c23 Jul 11, 2018
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ml [SPARK-23254][ML] Add user guide entry and example for DataFrame mult… Jul 11, 2018
mllib [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
sql [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
streaming [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
als.py [MINOR] Remove unused arg in als.py Jul 19, 2016
avro_inputformat.py [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
kmeans.py [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
logistic_regression.py [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
pagerank.py [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
parquet_inputformat.py [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
pi.py [SPARK-20779][EXAMPLES] The ASF header placed in an incorrect locatio… May 18, 2017
py_container_checks.py [SPARK-23984][K8S] Initial Python Bindings for PySpark on K8s Jun 8, 2018
pyfiles.py [SPARK-23984][K8S] Initial Python Bindings for PySpark on K8s Jun 8, 2018
sort.py [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018
status_api_demo.py [SPARK-19134][EXAMPLE] Fix several sql, mllib and status api examples… Jan 10, 2017
transitive_closure.py [SPARK-15773][CORE][EXAMPLE] Avoid creating local variable `sc` in ex… Jun 10, 2016
wordcount.py [SPARK-23522][PYTHON] always use sys.exit over builtin exit Mar 8, 2018