Document that app code should not pass 'dataset_id' to 'Key.__init__'.

Only the 'helpers.key_from_protobuf' factory should pass it.

Fixes #293.

Author: tseaver

gcloud/datastore/key.py

@@ -24,7 +24,9 @@ def __init__(self, path=None, namespace=None, dataset_id=None):
 
         :type dataset_id: string
         :param dataset: The dataset ID assigned by back-end for the key.
-                        Leave as None for newly-created keys.
+                        Application code should leave this field None;
+                        expected to be set only by
+                        :func:`gcloud.datastore.helprs.key_from_protobuf`.
         """
         self._path = path or [{'kind': ''}]
         self._namespace = namespace