Set demo app up with deferred help

Change-Id: Ia48866e17d5823a2ccc85172c717581316ad05a7

demoapp/cliffdemo/main.py

@@ -11,6 +11,7 @@ class DemoApp(App):
             description='cliff demo app',
             version='0.1',
             command_manager=CommandManager('cliff.demo'),
+            deferred_help=True,
             )
 
     def initialize_app(self, argv):

doc/source/demoapp.rst

@@ -66,6 +66,10 @@ To see help for an individual command, use the ``help`` command::
 
   (.venv)$ cliffdemo help files
 
+or the ``--help`` option::
+
+  (.venv)$ cliffdemo files --help
+
 The Source
 ==========