Show default values on the Documentation

In the documentation page for the [CatBoostRegressor](https://catboost.ai/en/docs/concepts/python-reference_catboostregressor) there are no default values of parameters or explanations on parameters shown except for the `loss_function` parameter. It is very hard to learn what a parameters does. Sometimes even adding the default help guess what are other viable values. For example you guess that the `loss_function='MAE'` parameter just expects the abbreviation of the name of the metric as a string.

Thank you :)

Start with the linked CatBoostRegressor documentation page and review its parameter reference, including loss_function. Add default values and explanations for the parameters shown there, then verify that the page makes valid values and defaults understandable.

Written by the indexing model from the issue text.