This is based on #4801 from dbtsai. The linear method guide is re-organized a little bit for this change.

Closes #4801

Author: Xiangrui Meng <meng@databricks.com>
Author: DB Tsai <dbtsai@alpinenow.com>

Closes #4861 from mengxr/SPARK-5537 and squashes the following commits:

47af0ac [Xiangrui Meng] update user guide for multinomial logistic regression
cdc2e15 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into AlpineNow-mlor-doc
096d0ca [DB Tsai] first commit