Add information to wake_losses_model parameter description

Author: SabineHaas

windpowerlib/wind_farm.py

@@ -293,8 +293,9 @@ def assign_power_curve(self, wake_losses_model='wind_farm_efficiency',
         ----------
         wake_losses_model : str
             Defines the method for taking wake losses within the farm into
-            consideration. Options: 'wind_farm_efficiency' or None.
-            Default: 'wind_farm_efficiency'.
+            consideration. Options: 'wind_farm_efficiency' or None. If
+            'wind_farm_efficiency' is chosen the `efficiency` of the WindFarm
+            must be set. Default: 'wind_farm_efficiency'.
         smoothing : bool
             If True the power curves will be smoothed before or after the
             aggregation of power curves depending on `smoothing_order`.

windpowerlib/wind_turbine_cluster.py

@@ -137,8 +137,10 @@ def assign_power_curve(self, wake_losses_model='wind_farm_efficiency',
         Parameters
         ----------
         wake_losses_model : str
-            Defines the method for taking wake losses within the farm into
-            consideration. Options: 'wind_farm_efficiency' or None.
+            Defines the method for taking wake losses within the wind farms of
+            the  cluster into consideration. Options: 'wind_farm_efficiency'
+            or None. If 'wind_farm_efficiency' is chosen the `efficiency`
+            attribute of the WindFarms must be set.
             Default: 'wind_farm_efficiency'.
         smoothing : bool
             If True the power curves will be smoothed before or after the