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 12, 2018