[docs] [dask] Add return information to Dask fit() docs (fixes #4402) (…

…#4716)

* [docs] [dask] Add return information to Dask fit() docs (fixes #4402)

* put return before note

python-package/lightgbm/dask.py

@@ -1213,6 +1213,11 @@ def fit(
     fit.__doc__ = f"""{_base_doc[:_base_doc.find('callbacks :')]}**kwargs
         Other parameters passed through to ``LGBMClassifier.fit()``.
 
+    Returns
+    -------
+    self : lightgbm.DaskLGBMClassifier
+        Returns self.
+
     {_lgbmmodel_doc_custom_eval_note}
         """
 
@@ -1393,6 +1398,11 @@ def fit(
     fit.__doc__ = f"""{_base_doc[:_base_doc.find('callbacks :')]}**kwargs
         Other parameters passed through to ``LGBMRegressor.fit()``.
 
+    Returns
+    -------
+    self : lightgbm.DaskLGBMRegressor
+        Returns self.
+
     {_lgbmmodel_doc_custom_eval_note}
         """
 
@@ -1556,6 +1566,11 @@ def fit(
     fit.__doc__ = f"""{_base_doc[:_base_doc.find('callbacks :')]}**kwargs
         Other parameters passed through to ``LGBMRanker.fit()``.
 
+    Returns
+    -------
+    self : lightgbm.DaskLGBMRanker
+        Returns self.
+
     {_lgbmmodel_doc_custom_eval_note}
         """