Port ansible doc for plugins to use DOCUMENTATION variables

* Using docstrings conflicts with the standard use of docstrings
* PYTHON_OPTIMIZE=2 will omit docstrings.  Using docstrings makes future
  changes to the plugin and module code subject to the requirement that we
  ensure it won't be run with optimization.

lib/ansible/plugins/callback/context_demo.py

@@ -2,8 +2,10 @@
 # (c) 2017 Ansible Project
 # GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
 
-'''
-DOCUMENTATION:
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+DOCUMENTATION = '''
     callback: context_demo
     type: aggregate
     short_description: demo callback that adds play/task context
@@ -15,9 +17,6 @@ DOCUMENTATION:
       - whitelist in configuration
 '''
 
-from __future__ import (absolute_import, division, print_function)
-__metaclass__ = type
-
 from ansible.plugins.callback import CallbackBase