[SPARK-15341][DOC][ML] Add documentation for "model.write" to clarify "summary" was not saved

## What changes were proposed in this pull request?
Currently in ```model.write```, we don't save ```summary```(if applicable). We should add documentation to clarify it.
We fixed the incorrect link ```[[MLWriter]]``` to ```[[org.apache.spark.ml.util.MLWriter]]``` BTW.

## How was this patch tested?
Documentation update, no unit test.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #13131 from yanboliang/spark-15341.