Document support for "checkpoint" in XGBoost

[exalate-issue-sync]

PUBDEV-6815 extends support for the "checkpoint" option in XGBoost. The following topics need to be updated as a result:

• https://github.com/h2oai/h2o-3/blob/master/h2o-docs/src/product/checkpointing-models.rst
• https://github.com/h2oai/h2o-3/blob/master/h2o-docs/src/product/data-science/xgboost.rst#defining-an-xgboost-model
• https://github.com/h2oai/h2o-3/blob/master/h2o-docs/src/product/data-science/algo-params/checkpoint.rst

XGBoost acts like DRF and GBM (uses ntrees).

Update, the pull request for the feature was changed to master, so this should go in 3.28.0.1.
-Note that PUBDEV-6815 is marked for 3.28.0.1, but its pull request is for rel-yau, so that's why I marked this 3.26.0.4.-

[exalate-issue-sync]

Angela Bartz commented: Merged pull request into master

[h2o-ops]

JIRA Issue Migration Info

Jira Issue: PUBDEV-6828
Assignee: hannah.tillman
Reporter: Angela Bartz
State: Resolved
Fix Version: 3.28.0.1
Attachments: N/A
Development PRs: Available

Linked PRs from JIRA

#3915