ML, Graph 2.3 QA: API: New Scala APIs, docs

Audit new public Scala APIs added to MLlib & GraphX. Take note of:

• Protected/public classes or methods. If access can be more private, then it should be.
• Also look for non-sealed traits.
• Documentation: Missing? Bad links or formatting?

Make sure to check the object doc!

As you find issues, please create JIRAs and link them to this issue. 

For user guide issues link the new JIRAs to the relevant user guide QA issue (SPARK-23111 for 2.3)