Move terminology definition to class related

As of now, the glossary defined in our documentation reflects the
current state of the codebase. In order to avoid any discrepancy
between the codebase and each definition, the objective here is to
gather both in a single place and link it into the rst documentation
via a custom directive.
Also re-aligned the requirements with liberty for doc.

Change-Id: I3089fd9f948b948115672f10937b1500b96ce180
Partial-Bug: #1526671

watcher/api/controllers/v1/audit.py

@@ -15,6 +15,40 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+"""
+In the Watcher system, an :ref:`Audit <audit_definition>` is a request for
+optimizing a :ref:`Cluster <cluster_definition>`.
+
+The optimization is done in order to satisfy one :ref:`Goal <goal_definition>`
+on a given :ref:`Cluster <cluster_definition>`.
+
+For each :ref:`Audit <audit_definition>`, the Watcher system generates an
+:ref:`Action Plan <action_plan_definition>`.
+
+An :ref:`Audit <audit_definition>` has a life-cycle and its current state may
+be one of the following:
+
+-  **PENDING** : a request for an :ref:`Audit <audit_definition>` has been
+   submitted (either manually by the
+   :ref:`Administrator <administrator_definition>` or automatically via some
+   event handling mechanism) and is in the queue for being processed by the
+   :ref:`Watcher Decision Engine <watcher_decision_engine_definition>`
+-  **ONGOING** : the :ref:`Audit <audit_definition>` is currently being
+   processed by the
+   :ref:`Watcher Decision Engine <watcher_decision_engine_definition>`
+-  **SUCCEEDED** : the :ref:`Audit <audit_definition>` has been executed
+   successfully (note that it may not necessarily produce a
+   :ref:`Solution <solution_definition>`).
+-  **FAILED** : an error occured while executing the
+   :ref:`Audit <audit_definition>`
+-  **DELETED** : the :ref:`Audit <audit_definition>` is still stored in the
+   :ref:`Watcher database <watcher_database_definition>` but is not returned
+   any more through the Watcher APIs.
+-  **CANCELLED** : the :ref:`Audit <audit_definition>` was in **PENDING** or
+   **ONGOING** state and was cancelled by the
+   :ref:`Administrator <administrator_definition>`
+"""
+
 import datetime
 
 import pecan