Update 'gcloud.datastore.helpers' to indicate API-ness.

Correct docstring.

Add '__all__' to indicate public members.

Author: tseaver

gcloud/datastore/helpers.py

@@ -1,7 +1,9 @@
 """Helper functions for dealing with Cloud Datastore's Protobuf API.
 
-These functions are *not* part of the API.
+The non-private functions are part of the API.
 """
+__all__ = ('entity_from_protobuf', 'key_from_protobuf')
+
 import calendar
 import datetime