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/timer.py

@@ -1,7 +1,11 @@
 # (c) 2017 Ansible Project
 # GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
-'''
-DOCUMENTATION:
+
+# Make coding more python3-ish
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+DOCUMENTATION = '''
     callback: timer
     callback_type: aggregate
     requirements:
@@ -11,9 +15,6 @@ DOCUMENTATION:
     description:
         - This callback just adds total play duration to the play stats.
 '''
-# Make coding more python3-ish
-from __future__ import (absolute_import, division, print_function)
-__metaclass__ = type
 
 from datetime import datetime