add docstring to _set_protobuf_property, remove docstring related to …

…None value

gcloud/datastore/connection.py

@@ -430,8 +430,8 @@ def save_entity(self, dataset_id, key_pb, properties,
            not passed in ``properties`` no longer be set for the entity.
 
         .. note::
-           When saving an entity to the backend, property value set as
-           empty lists or None cannot be saved and will be ignored.
+           When saving an entity to the backend, a property value set as
+           an empty list cannot be saved and will be ignored.
 
         :type dataset_id: string
         :param dataset_id: The ID of the dataset in which to save the entity.

gcloud/datastore/helpers.py

@@ -219,6 +219,27 @@ def _get_value_from_property_pb(property_pb):
 
 
 def _set_protobuf_property(property_pb, name, value, indexed):
+    """Assign 'name', 'value', 'indexed' to the correct 'property_pb'.
+
+    Some value (empty list) cannot be directly assigned; this function handles
+    them correctly.
+
+    :type property_pb: :class:`gcloud.datastore.datastore_v1_pb2.Property`
+    :param property_pb: The value protobuf to which the value is being
+                        assigned.
+
+    :type name: string
+    :param name: The name to be assigned.
+
+    :type value: `datetime.datetime`, boolean, float, integer, string,
+                 :class:`gcloud.datastore.key.Key`,
+                 :class:`gcloud.datastore.entity.Entity`,
+    :param value: The value to be assigned.
+
+    :type indexed: boolean
+    :param indexed: The flag indicates the property should to be indexed or
+                    not.
+    """
     if isinstance(value, list) and len(value) == 0:
         return