Add Missing Attributes to `KhiopsRegressor`

[popescu-v]

Description

Currently, KhiopsRegressor lacks documentation for n_trees, n_text_features and type_text_features, even though:

• these attributes are present on the estimator itself and can be set in KhiopsRegressor.__init__.
• the passage of these attributes to the Core API is fully tested for the regressor as well.

Questions/Ideas

• These attributes must be added to KhiopsRegressor's docstring.