Enable documentation in plugins

Made ansible-doc more plugin agnostic
We can have docs in lookup, callback, connectionm strategy, etc
Use first docstring and make pepizis happy
generalized module_docs to plugin_docs
documented cartesian, ssh, default, jsonfile, etc as examples
changed lack of docs to warning when listing
made smarter about bad docstrings
better blacklisting
added handling of options/config/envs/etc
move blacklist to find_plugins, only need once

lib/ansible/plugins/strategy/linear.py

@@ -14,7 +14,19 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
-
+'''
+DOCUMENTATION:
+    strategy: linear
+    short_description: Executes tasks in a linear fashion
+    description:
+        - Task execution is in lockstep per host batch as defined by C(serial) (default all).
+          Up to the fork limit of hosts will execute each task at the same time and then
+          the next series of hosts until the batch is done, before going on to the next task.
+    version_added: "2.0"
+    notes:
+     - This was the default Ansible behaviour before 'strategy plugins' were introduces in 2.0.
+    author: Ansible Core Team
+'''
 # Make coding more python3-ish
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type