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/selective.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: selective
     callback_type: stdout
     requirements:
@@ -26,21 +28,20 @@ DOCUMENTATION:
           - section: defaults
           - key: nocolor
         type: boolean
-EXAMPLES:
+"""
+
+EXAMPLES = """
   - debug: msg="This will not be printed"
   - debug: msg="But this will"
     tags: [print_action]
 """
 
-from __future__ import (absolute_import, division, print_function)
-
 import difflib
 
 from ansible import constants as C
 from ansible.plugins.callback import CallbackBase
 from ansible.module_utils._text import to_text
 
-__metaclass__ = type
 
 DONT_COLORIZE = False
 COLORS = {