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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graphx [SPARK-22896] Improvement in String interpolation Jan 3, 2018
ml [SPARK-23254][ML] Add user guide entry and example for DataFrame mult… Jul 11, 2018
mllib [SPARK-22896] Improvement in String interpolation Jan 3, 2018
pythonconverters [SPARK-21731][BUILD] Upgrade scalastyle to 0.9. Aug 15, 2017
sql [SPARK-22833][EXAMPLE] Improvement SparkHive Scala Examples Dec 26, 2017
streaming [SPARK-22968][DSTREAM] Throw an exception on partition revoking issue Apr 18, 2018
BroadcastTest.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
DFSReadWriteTest.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
DriverSubmissionTest.scala [SPARK-14444][BUILD] Add a new scalastyle `NoScalaDoc` to prevent Sca… Apr 6, 2016
ExceptionHandlingTest.scala [SPARK-15773][CORE][EXAMPLE] Avoid creating local variable `sc` in ex… Jun 10, 2016
GroupByTest.scala [SPARK-15773][CORE][EXAMPLE] Avoid creating local variable `sc` in ex… Jun 10, 2016
HdfsTest.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
LocalALS.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
LocalFileLR.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
LocalKMeans.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
LocalLR.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
LocalPi.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
LogQuery.scala [SPARK-13583][CORE][STREAMING] Remove unused imports and add checksty… Mar 3, 2016
MultiBroadcastTest.scala [MINOR] Issue: Change "slice" vs "partition" in exception messages (a… Apr 9, 2017
SimpleSkewedGroupByTest.scala [SPARK-22896] Improvement in String interpolation Jan 3, 2018
SkewedGroupByTest.scala [SPARK-15773][CORE][EXAMPLE] Avoid creating local variable `sc` in ex… Jun 10, 2016
SparkALS.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
SparkHdfsLR.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
SparkKMeans.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
SparkLR.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
SparkPageRank.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
SparkPi.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017
SparkRemoteFileTest.scala [SPARK-22839][K8S] Remove the use of init-container for downloading r… Mar 19, 2018
SparkTC.scala [SPARK-22830] Scala Coding style has been improved in Spark Examples Dec 20, 2017