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/mail.py

@@ -2,8 +2,10 @@
 # Copyright: (c) 2012, Dag Wieers <dag@wieers.com>
 # 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: mail
     type: notification
     short_description: Sends failure events via email
@@ -23,9 +25,6 @@ DOCUMENTATION:
       - "TODO: expand configuration options now that plugins can leverage Ansible's configuration"
 '''
 
-from __future__ import (absolute_import, division, print_function)
-__metaclass__ = type
-
 import json
 import os
 import smtplib