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/default.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: default
     type: stdout
     short_description: default Ansible screen output
@@ -35,9 +37,6 @@ DOCUMENTATION:
       - set as stdout in configuration
 '''
 
-from __future__ import (absolute_import, division, print_function)
-__metaclass__ = type
-
 from ansible import constants as C
 from ansible.playbook.task_include import TaskInclude
 from ansible.plugins.callback import CallbackBase