SparkR 2.1 QA: Update user guide for new features & APIs

Check the user guide vs. a list of new APIs (classes, methods, data members) to see what items require updates to the user guide.

For each feature missing user guide doc:

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

If you would like to work on this task, please comment, and we can create & link JIRAs for parts of this work.