[SPARK-4583] [mllib] LogLoss for GradientBoostedTrees fix + doc updates

Currently, the LogLoss used by GradientBoostedTrees has 2 issues:
* the gradient (and therefore loss) does not match that used by Friedman (1999)
* the error computation uses 0/1 accuracy, not log loss

This PR updates LogLoss.
It also adds some doc for boosting and forests.

I tested it on sample data and made sure the log loss is monotonically decreasing with each boosting iteration.

CC: @mengxr @manishamde
`MSE` is not usually defined with multiplier `1/2`. Shall we use a different name here, or example, `mean squared loss` or `average loss`?