[SPARK-5537][MLlib][Docs] Add user guide for multinomial logistic regression
[SPARK-5537][MLlib][Docs] Add user guide for multinomial logistic regression

Adding more description on top of #4861.

Author: DB Tsai <dbtsai@alpinenow.com>

Closes #4866 from dbtsai/doc and squashes the following commits:

37e9d07 [DB Tsai] doc

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