Documentation update for get-goal-from-strategy

In this changeset, I updated the Watcher documentation to reflect
the changes that are introduced by this blueprint.

Partially Implements: blueprint get-goal-from-strategy

Change-Id: I40be39624097365220bf7d94cbe177bbf5bbe0ed

watcher/api/controllers/v1/strategy.py

@@ -14,6 +14,19 @@
 # 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 goal.
+
+Some strategies may provide better optimization results but may take more time
+to find an optimal :ref:`Solution <solution_definition>`.
+"""
+
 from oslo_config import cfg
 
 import pecan