Correct instance parameter description

get_node_by_instance() only support for 'instance uuid', remove
'instance name' to avoid confusion.

Change-Id: Icaebf43c5daeba055b6d6054b1a51f4a51468620

ironic/db/api.py

@@ -174,8 +174,10 @@ class Connection(object):
     def get_node_by_instance(self, instance):
         """Return a node.
 
-        :param instance: The instance name or uuid to search for.
+        :param instance: The instance uuid to search for.
         :returns: A node.
+        :raises: InstanceNotFound if the instance is not found.
+        :raises: InvalidUUID if the instance uuid is invalid.
         """
 
     @abc.abstractmethod