Target Encoding data_leakage_handling parameter values updated in R to match Python

[exalate-issue-sync]

Some follow-up tasks to the big TE refactoring/API update PR: [https://github.com//pull/4816|https://github.com//pull/4816]

data_leakage_handling consistent param value names in Py/R (enable both syntax on both clients); use the Python names in docs/examples/tests

• none (Python)/None (R): Do not holdout anything. Using whole frame for training
• k_fold (Python)/KFold (R): Encodings for a fold are generated based on out-of-fold data.
• leave_one_out (Python)/LeaveOneOut (R): The current row’s response value is subtracted from the pre-calculated per-level frequencies.

2. Add backwards compatibility R tests.
3. Also add default values for all the arguments in [TE user guide page|http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-munging/target-encoding.html].
4. Lastly, clean up any other code on the user guide page that is now outdated.

[h2o-ops]

JIRA Issue Migration Info

Jira Issue: PUBDEV-7749
Assignee: Sebastien Poirier
Reporter: Erin LeDell
State: Resolved
Fix Version: 3.32.0.2
Attachments: N/A
Development PRs: Available

Linked PRs from JIRA

#5124