Improve default threshold API

[maurever]

Based on this issue #15712, I suggested improving our API and also doc here: https://docs.h2o.ai/h2o/latest-stable/h2o-docs/performance-and-prediction.html#prediction-threshold

• Improve Python and R API to get default threshold (to call model.defautl_threshold())

These part will be implemented later:

• Explain how threshold affects prediction
• Add an example of how to get optimal thresholds for all calculated metrics (using the thresholds_and_metric_scores table)
• Add an example of how to change the default threshold (using the reset_model_threshold function, examples here: Python and R)

[maurever]

@hannah-tillman, can you please take a look? More info in #15712. Users often ask us this question. I found out, there is no documentation on how to change the default threshold.

[maurever]

@hannah-tillman, I implemented the code part of this issue here: #15714.

[maurever]

@hannah-tillman the API change is merged.

[maurever]

This issue solves only the implementation part.