DOC clarify StratifiedShuffleSplit supports 2d y (scikit-learn#15456)

Author: edvardlindelof

sklearn/model_selection/_split.py

@@ -1713,7 +1713,7 @@ def split(self, X, y, groups=None):
             hence ``np.zeros(n_samples)`` may be used as a placeholder for
             ``X`` instead of actual training data.
 
-        y : array-like, shape (n_samples,)
+        y : array-like, shape (n_samples,) or (n_samples, n_labels)
             The target variable for supervised learning problems.
             Stratification is done based on the y labels.