[SPARK-18633][ML][EXAMPLE] Add multiclass logistic regression summary python example and document

## What changes were proposed in this pull request?
Logistic Regression summary is added in Python API. We need to add example and document for summary.

The newly added example is consistent with Scala and Java examples.

## How was this patch tested?

Manually tests: Run the example with spark-submit; copy & paste code into pyspark; build document and check the document.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #16064 from wangmiao1981/py.