## What changes were proposed in this pull request?

stop session at end of example

## How was this patch tested?

manual

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16973 from felixcheung/rexamples.

## What changes were proposed in this pull request?
Update programming guide, example and vignette with Bisecting k-means.

Author: krishnakalyan3 <krishnakalyan3@gmail.com>

Closes #16767 from krishnakalyan3/bisecting-kmeans.

## What changes were proposed in this pull request?
```ml.R``` example depends on ```e1071``` package, if it's not available in users' environment, it will fail. I think the example should not depends on third-party packages, so I update it to remove the dependency.

## How was this patch tested?
Manual test.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #16548 from yanboliang/spark-19158.

## What changes were proposed in this pull request?
* Add all R examples for ML wrappers which were added during 2.1 release cycle.
* Split the whole ```ml.R``` example file into individual example for each algorithm, which will be convenient for users to rerun them.
* Add corresponding examples to ML user guide.
* Update ML section of SparkR user guide.

Note: MLlib Scala/Java/Python examples will be consistent, however, SparkR examples may different from them, since R users may use the algorithms in a different way, for example, using R ```formula``` to specify ```featuresCol``` and ```labelCol```.

## How was this patch tested?
Run all examples manually.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #16148 from yanboliang/spark-18325.