Apply suggestions from code review

Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>

Author: cwhanse

pvlib/pvsystem.py

@@ -1215,7 +1215,7 @@ def get_iam(self, aoi, iam_model='physical'):
             raise ValueError(model + ' is not a valid IAM model')
 
     def get_cell_temperature(self, poa_global, temp_air, wind_speed, model,
-                             effective_irradiance=None, ir_down=None):
+                             effective_irradiance=None, longwave_down=None):
         """
         Determine cell temperature using the method specified by ``model``.
 
@@ -1240,7 +1240,7 @@ def get_cell_temperature(self, poa_global, temp_air, wind_speed, model,
             The irradiance that is converted to photocurrent in W/m^2.
             Only used for some models.
 
-        ir_down: numeric, optional
+        longwave_down: numeric, optional
             Downwelling infrared radiation from the sky, measured on a
             horizontal surface in W/m^2. Only used in ``'faiman_rad'`` model.
 
@@ -1287,7 +1287,7 @@ def get_cell_temperature(self, poa_global, temp_air, wind_speed, model,
                                      self.temperature_model_parameters)
         elif model == 'faiman_rad':
             func = functools.partial(temperature.faiman_rad,
-                                     ir_down=ir_down)
+                                     ir_down=longwave_down)
             required = ()
             optional = _build_kwargs(['u0', 'u1',
                                       'sky_view', 'emissivity'],