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/lookup/cartesian.py

@@ -14,6 +14,34 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
+"""
+DOCUMENTATION:
+    lookup: cartesian
+    version_added: "2.1"
+    short_description: returns the cartesian product of lists
+    description:
+        - Takes the input lists and returns a list that represents the product of the input lists.
+    options:
+        _raw:
+            description:
+                - a set of lists
+            required: True
+EXAMPLES:
+
+  - name: outputs the cartesian product of the supplied lists
+    debug: msg="{{item}}"
+    with_cartesian:
+         - "{{list1}}"
+         - "{{list2}}"
+  - name: used as lookup changes [1, 2, 3], [a, b] into [1, a], [1, b], [2, a], [2, b], [3, a], [3, b]
+    debug: msg="{{ [1,2,3]|lookup('cartesian', [a, b])}}"
+
+RETURN:
+  _list:
+    description:
+      - list of lists composed of elements of the input lists
+    type: lists
+"""
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type