[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
I'll remove the 1/2.  It's probably better to have an odd loss (which only experts need to know about) than to have an odd name (which everyone needs to recognize).