[SPARK-10061] [DOC] ML ensemble docs

[jkbradley]

User guide for spark.ml GBTs and Random Forests.
The examples are copied from the decision tree guide and modified to run.

I caught some issues I had somehow missed in the tree guide as well.

I have run all examples, including Java ones. (Of course, I thought I had previously as well...)

CC: @mengxr @manishamde @yanboliang

[jkbradley]

Note: This is blocked by [https://github.com//pull/8367]

[SparkQA]

Test build #41393 has finished for PR 8369 at commit e054358.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • * a bit more functionality for random forests: estimates of feature importance, as well as the predicted probability of each class (a.k.a. class conditional probabilities) for classification.
  • [OneVsRest](http://en.wikipedia.org/wiki/Multiclass_classification#One-vs.-rest) is an example of a machine learning reduction for performing multiclass classification given a base classifier that can perform binary classification efficiently. It is also known as "One-vs-All."

[jkbradley]

Rebased to include GBT fix in master.

[SparkQA]

Test build #41395 has finished for PR 8369 at commit 42f4f2a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • * a bit more functionality for random forests: estimates of feature importance, as well as the predicted probability of each class (a.k.a. class conditional probabilities) for classification.
  • [OneVsRest](http://en.wikipedia.org/wiki/Multiclass_classification#One-vs.-rest) is an example of a machine learning reduction for performing multiclass classification given a base classifier that can perform binary classification efficiently. It is also known as "One-vs-All."
  • public class JavaVectorSlicerSuite

[SparkQA]

Test build #41433 has finished for PR 8369 at commit 5598f46.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[yanboliang]

Looks good!

[mengxr]

space after []

[mengxr]

LGTM except some minor comments.

[jkbradley]

OK I think that does it. Thanks for reviewing!

[SparkQA]

Test build #41476 has finished for PR 8369 at commit e1c6e18.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

Merged into master and branch-1.5. Thanks!