[SPARK-2841][MLlib] Documentation for feature transformations

Documentation for newly added feature transformations:
1. TF-IDF
2. StandardScaler
3. Normalizer
How about I say
"For example, RBF kernel of Support Vector Machines
or the L1 and L2 regularized linear models typically works better when all features have unit variance
and/or zero mean."

I actually have this statement from scikit documentation.  
http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.StandardScaler.html