User guide for isotonic regression added to docs/mllib-regression.md including code examples for Scala and Java.

Author: martinzapletal <zapletal-martin@email.cz>

Closes #4536 from zapletal-martin/SPARK-5502 and squashes the following commits:

67fe773 [martinzapletal] SPARK-5502 reworded model prediction rules to use more general language rather than the code/implementation specific terms
80bd4c3 [martinzapletal] SPARK-5502 created docs page for isotonic regression, added links to the page, updated data and examples
7d8136e [martinzapletal] SPARK-5502 Added documentation for Isotonic regression including examples for Scala and Java
504b5c3 [martinzapletal] SPARK-5502 Added documentation for Isotonic regression including examples for Scala and Java