corrected the code comment for the output of find_pruneable_heads_and_indices (#22557)

* corrected/clarified the code comment of find_pruneable_heads_and_indices

* have run make style

src/transformers/pytorch_utils.py

@@ -249,7 +249,8 @@ def find_pruneable_heads_and_indices(
         already_pruned_heads (`Set[int]`): A set of already pruned heads.
 
     Returns:
-        `Tuple[Set[int], torch.LongTensor]`: A tuple with the remaining heads and their corresponding indices.
+        `Tuple[Set[int], torch.LongTensor]`: A tuple with the indices of heads to prune taking `already_pruned_heads`
+        into account and the indices of rows/columns to keep in the layer weight.
     """
     mask = torch.ones(n_heads, head_size)
     heads = set(heads) - already_pruned_heads  # Convert to set and remove already pruned heads