[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 <[email protected]>

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

ec3c989 [DB Tsai] first commit

Author: DB Tsai

mllib/src/main/scala/org/apache/spark/mllib/regression/Lasso.scala

@@ -45,7 +45,7 @@ class LassoModel (
 /**
  * Train a regression model with L1-regularization using Stochastic Gradient Descent.
  * This solves the l1-regularized least squares regression formulation
- *          f(weights) = 1/n ||A weights-y||^2  + regParam ||weights||_1
+ *          f(weights) = 1/2n ||A weights-y||^2  + regParam ||weights||_1
  * Here the data matrix has n rows, and the input RDD holds the set of rows of A, each with
  * its corresponding right hand side label y.
  * See also the documentation for the precise formulation.

mllib/src/main/scala/org/apache/spark/mllib/regression/RidgeRegression.scala

@@ -45,7 +45,7 @@ class RidgeRegressionModel (
 /**
  * Train a regression model with L2-regularization using Stochastic Gradient Descent.
  * This solves the l1-regularized least squares regression formulation
- *          f(weights) = 1/n ||A weights-y||^2  + regParam/2 ||weights||^2
+ *          f(weights) = 1/2n ||A weights-y||^2  + regParam/2 ||weights||^2
  * Here the data matrix has n rows, and the input RDD holds the set of rows of A, each with
  * its corresponding right hand side label y.
  * See also the documentation for the precise formulation.