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/decision_engine/strategy/strategies/base.py

@@ -14,6 +14,24 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+"""
+A :ref:`Strategy <strategy_definition>` is an algorithm implementation which is
+able to find a :ref:`Solution <solution_definition>` for a given
+:ref:`Goal <goal_definition>`.
+
+There may be several potential strategies which are able to achieve the same
+:ref:`Goal <goal_definition>`. This is why it is possible to configure which
+specific :ref:`Strategy <strategy_definition>` should be used for each
+:ref:`Goal <goal_definition>`.
+
+Some strategies may provide better optimization results but may take more time
+to find an optimal :ref:`Solution <solution_definition>`.
+
+When a new :ref:`Goal <goal_definition>` is added to the Watcher configuration,
+at least one default associated :ref:`Strategy <strategy_definition>` should be
+provided as well.
+"""
+
 import abc
 from oslo_log import log
 import six