Add **kwargs argument description for models

CHANGELOG.md

@@ -7,7 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 ### Added
-- 
+- Add `**kwargs` argument description for models based on `LinearRegression`, `ElasticNet` and `CatBoostRegressor` ([#454](https://github.com/etna-team/etna/pull/454https://github.com/etna-team/etna/pull/454))  
 - 
 - 
 - 

etna/models/catboost.py

@@ -283,6 +283,8 @@ def __init__(
             * For GPU. The given value is used for reading the data from the hard drive and does
               not affect the training.
               During the training one main thread and one thread for each GPU are used.
+        **kwargs:
+            Additional parameters passed to :py:class:`catboost.CatBoostRegressor` model.
         """
         self.iterations = iterations
         self.depth = depth
@@ -422,6 +424,8 @@ def __init__(
             * For GPU. The given value is used for reading the data from the hard drive and does
               not affect the training.
               During the training one main thread and one thread for each GPU are used.
+        **kwargs:
+            Additional parameters passed to :py:class:`catboost.CatBoostRegressor` model.
         """
         self.iterations = iterations
         self.depth = depth

etna/models/linear.py

@@ -72,6 +72,8 @@ def __init__(self, fit_intercept: bool = True, **kwargs):
         fit_intercept:
             Whether to calculate the intercept for this model. If set to False, no intercept will be used in
             calculations (i.e. data is expected to be centered).
+        **kwargs:
+            Additional parameters passed to :py:class:`sklearn.linear_model.LinearRegression` model.
         """
         self.fit_intercept = fit_intercept
         self.kwargs = kwargs
@@ -126,6 +128,8 @@ def __init__(self, alpha: float = 1.0, l1_ratio: float = 0.5, fit_intercept: boo
         fit_intercept:
             Whether to calculate the intercept for this model. If set to False, no intercept will be used in
             calculations (i.e. data is expected to be centered).
+        **kwargs:
+            Additional parameters passed to :py:class:`sklearn.linear_model.ElasticNet` model.
         """
         self.alpha = alpha
         self.l1_ratio = l1_ratio
@@ -175,6 +179,8 @@ def __init__(self, fit_intercept: bool = True, **kwargs):
         fit_intercept:
             Whether to calculate the intercept for this model. If set to False, no intercept will be used in
             calculations (i.e. data is expected to be centered).
+        **kwargs:
+            Additional parameters passed to :py:class:`sklearn.linear_model.LinearRegression` model.
         """
         self.fit_intercept = fit_intercept
         self.kwargs = kwargs
@@ -229,6 +235,8 @@ def __init__(self, alpha: float = 1.0, l1_ratio: float = 0.5, fit_intercept: boo
         fit_intercept:
             Whether to calculate the intercept for this model. If set to False, no intercept will be used in
             calculations (i.e. data is expected to be centered).
+        **kwargs:
+            Additional parameters passed to :py:class:`sklearn.linear_model.ElasticNet` model.
         """
         self.alpha = alpha
         self.l1_ratio = l1_ratio