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/debug.py

@@ -1,8 +1,10 @@
 # (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: debug
     type: stdout
     short_description: formated stdout/stderr display
@@ -12,8 +14,6 @@ DOCUMENTATION:
     requirements:
       - set as stdout in configuration
 '''
-from __future__ import (absolute_import, division, print_function)
-__metaclass__ = type
 
 from ansible.plugins.callback.default import CallbackModule as CallbackModule_default