[SPARK-4972][MLlib] Updated the scala doc for lasso and ridge regression for the change of LeastSquaresGradient

In #SPARK-4907, we added factor of 2 into the LeastSquaresGradient. We updated the scala doc for lasso and ridge regression here.

Author: DB Tsai <dbtsai@alpinenow.com>

Closes #3808 from dbtsai/doc and squashes the following commits:

ec3c989 [DB Tsai] first commit