Clarify docstring for 'Entity._must_key'.

Incorporates feedback from @dhermes.

Author: tseaver

gcloud/datastore/entity.py

@@ -168,7 +168,7 @@ def from_protobuf(cls, pb, dataset=None):  # pylint: disable=invalid-name
 
     @property
     def _must_key(self):
-        """Return our key.
+        """Return our key, or raise NoKey if not set.
 
         :rtype: :class:`gcloud.datastore.key.Key`.
         :returns: our key