Ensure docstring on storage properties

To ensure that sphinx generates docs for these
methods and shows them (without docstrings they
do not show up) add some basic docstrings on the
exposed storage classes properties.

Change-Id: Iab2d2432403c7065fe8cb679e8e64a24ec17b53f

taskflow/storage.py

@@ -263,16 +263,19 @@ class Storage(object):
 
     @property
     def flow_name(self):
+        """The flow detail name this storage unit is associated with."""
         # This never changes (so no read locking needed).
         return self._flowdetail.name
 
     @property
     def flow_uuid(self):
+        """The flow detail uuid this storage unit is associated with."""
         # This never changes (so no read locking needed).
         return self._flowdetail.uuid
 
     @property
     def backend(self):
+        """The backend this storage unit is associated with."""
         # This never changes (so no read locking needed).
         return self._backend