Additional fixes to Mllib 2.0 documentation

While reviewing the documentation of MLlib, I found some additional issues.

Important issues that affect the binary signatures:

• GBTClassificationModel: all the setters should be overriden
• LogisticRegressionModel: setThreshold(s)
• RandomForestClassificationModel: all the setters should be overriden
• org.apache.spark.ml.stat.distribution.MultivariateGaussian is exposed but most of the methods are private[ml] -> do we need to expose this class for now?
• GeneralizedLinearRegressionModel: linkObj, familyObj, familyAndLink should not be exposed
• sqlDataTypes: name does not follow conventions. Do we need to expose it?

Issues that involve only documentation:

• Evaluator:
  1. inconsistent doc between evaluate and isLargerBetter
• MinMaxScaler: math rendering
• GeneralizedLinearRegressionSummary: aic doc is incorrect

The reference documentation that was used was:
http://people.apache.org/~pwendell/spark-releases/spark-2.0.0-rc2-docs/