Add docstring.

keras/engine/topology.py

@@ -2716,6 +2716,18 @@ def save_weights_to_hdf5_group(f, layers):
 def preprocess_weights_for_loading(layer, weights,
                                    original_keras_version=None,
                                    original_backend=None):
+    """Converts layers weights from Keras 1 format to Keras 2.
+
+    # Arguments
+        layer: Layer instance.
+        weights: List of weights values (Numpy arrays).
+        original_keras_version: Keras version for the weights, as a string.
+        original_backend: Keras backend the weights were trained with,
+            as a string.
+
+    # Returns
+        A list of weights values (Numpy arrays).
+    """
     if original_keras_version == '1':
         if layer.__class__.__name__ == 'Conv1D':
             shape = weights[0].shape