[SPARK-18325][SPARKR][ML] SparkR ML wrappers example code and user guide

## 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.

1 files changed, 8 insertions, 0 deletions

diff --git a/docs/ml-collaborative-filtering.md b/docs/ml-collaborative-filtering.md
index 4d19b4069a..cfe835172a 100644
--- a/docs/ml-collaborative-filtering.md
+++ b/docs/ml-collaborative-filtering.md
@@ -149,4 +149,12 @@ als = ALS(maxIter=5, regParam=0.01, implicitPrefs=True,
 {% endhighlight %}
 
 </div>
+
+<div data-lang="r" markdown="1">
+
+Refer to the [R API docs](api/R/spark.als.html) for more details.
+
+{% include_example r/ml/als.R %}
+</div>
+
 </div>