[BRANCH-1.2][SPARK-4583][MLLIB] LogLoss for GradientBoostedTrees fix + doc updates

We reverted #3439 in branch-1.2 due to missing `import o.a.s.SparkContext._`, which is no longer needed in master (#3262). This PR adds #3439 back to branch-1.2 with correct imports.

Github is out-of-sync now. The real changes are the last two commits.