## What changes were proposed in this pull request?

There has been some confusion around "Spark ML" vs. "MLlib". This PR adds some FAQ-like entries to the MLlib user guide to explain "Spark ML" and reduce the confusion.

I check the [Spark FAQ page](http://spark.apache.org/faq.html), which seems too high-level for the content here. So I added it to the MLlib user guide instead.

cc: mateiz

Author: Xiangrui Meng <meng@databricks.com>

Closes #16241 from mengxr/SPARK-18812.