Apply suggestions from code review

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

Author: cwhanse

pvlib/pvsystem.py

@@ -414,7 +414,7 @@ def get_iam(self, aoi, iam_model='physical'):
 
     @_unwrap_single_value
     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``.
 
@@ -438,7 +438,7 @@ 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 or tuple of numeric, optional
+        longwave_down: numeric or tuple of numeric, optional
             Downwelling infrared radiation from the sky, measured on a
             horizontal surface in W/m^2. Only used in ``'faiman_rad'`` model.
 
@@ -464,7 +464,7 @@ def get_cell_temperature(self, poa_global, temp_air, wind_speed, model,
         # Not used for all models, but Array.get_cell_temperature handles it
         effective_irradiance = self._validate_per_array(effective_irradiance,
                                                         system_wide=True)
-        ir_down = self._validate_per_array(ir_down, system_wide=True)
+        longwave_down= self._validate_per_array(longwave_down, system_wide=True)
 
         return tuple(
             array.get_cell_temperature(poa_global, temp_air, wind_speed,