PUBDEV-4600: Documentation improvements for XGBoost (#1446)
PUBDEV-4600: Documentation improvements for XGBoost (#1446)

* PUBDEV-4600: Documentation improvements for XGBoost

- Added more information describing XGBoost
- Updated the parameters based on recent changes
- Updated the Limitations section
- Added a second reference.
Additional updates are still required. Those can be made in a future
release.

* PUBDEV-4600: XGBoost documentation updates

- Added more info about “LightGBM”
- Changed num_leaves to max_leaves and changed the default value to 0.
- Changed the default value for reg_lambda to 0.
- Added lambda_bias parameter.

* PUBDEV-4600: Doc updates for XGBoost

Added more clarification for when users can specify the light gbm
options.

mathemage/h2o-3,spennihana/h2o-3,h2oai/h2o-3,mathemage/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,mathemage/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,spennihana/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,mathemage/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,mathemage/h2o-3,h2oai/h2o-dev,mathemage/h2o-3,spennihana/h2o-3,h2oai/h2o-3,spennihana/h2o-3,spennihana/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,michalkurka/h2o-3,h2oai/h2o-3,mathemage/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,h2oai/h2o-3,h2oai/h2o-3