add insulated racking model option

Author: RDaxini

pvlib/pvsystem.py

@@ -168,8 +168,8 @@ class PVSystem:
         Inverter parameters as defined by the SAPM, CEC, or other.
 
     racking_model : string, optional
-        Valid strings are 'open_rack', 'close_mount', 'freestanding', or
-        'insulated_back'.
+        Valid strings are 'open_rack', 'close_mount', 'freestanding',
+        `insulated`, or 'insulated_back'.
         Used to identify a parameter set for the cell temperature model.
 
     losses_parameters : dict or Series, optional
@@ -1372,8 +1372,8 @@ class FixedMount(AbstractMount):
         West=270. [degrees]
 
     racking_model : str, optional
-        Valid strings are 'open_rack', 'close_mount', 'freestanding', or
-        'insulated_back'.
+        Valid strings are 'open_rack', 'close_mount', 'freestanding',
+        `insulated`, or 'insulated_back'.
         Used to identify a parameter set for the cell temperature model.
 
     module_height : float, optional