jira: https://issues.apache.org/jira/browse/SPARK-8531

Update ML user guide for MinMaxScaler

Author: Yuhao Yang <hhbyyh@gmail.com>
Author: unknown <yuhaoyan@yuhaoyan-MOBL1.ccr.corp.intel.com>

Closes #7211 from hhbyyh/minmaxdoc.