[FIX][DOC] Fix broken links in ml-guide.md

and some minor changes in ScalaDoc.

Author: Xiangrui Meng <meng@databricks.com>

Closes #3601 from mengxr/SPARK-4575-fix and squashes the following commits:

c559768 [Xiangrui Meng] minor code update
ce94da8 [Xiangrui Meng] Java Bean -> JavaBean
0b5c182 [Xiangrui Meng] fix links in ml-guide