SparkR 2.1 QA: Check for new R APIs requiring example code

Audit list of new features added to MLlib's R API, and see which major items are missing example code (in the examples folder). We do not need examples for everything, only for major items such as new algorithms.

For any such items:

• Create a JIRA for that feature, and assign it to the author of the feature (or yourself if interested).
• Link it to (a) the original JIRA which introduced that feature ("related to") and (b) to this JIRA ("requires").