Correct cloud_cover_to_transmittance_linear documentation (#1370)

* Modify cloud_cover_to_transmittance_linear documentation replacing ghi with transmittance

* Add documentation lines into v0.9.1.rst

* Modify line length in documentation

* Small modifs doc forecast.py

* Small modifs doc forecast.py

* Update pvlib/forecast.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>

* Rewording documentation forecast.py

Co-authored-by: Johann Loux <jloux@∂exma.com>
Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>

Author: 3 people

docs/sphinx/source/whatsnew/v0.9.1.rst

@@ -30,6 +30,9 @@ Testing
 
 Documentation
 ~~~~~~~~~~~~~
+* Fix documentation return error in :py:meth:`pvlib.forecast.ForecastModel.cloud_cover_to_transmittance_linear`
+  (:issue:`1367`, :pull:`1370`)
+
 
 Requirements
 ~~~~~~~~~~~~
@@ -42,3 +45,4 @@ Contributors
 * Christian Weickhmann (:ghuser:`cweickhmann`)
 * Kevin Anderson (:ghuser:`kanderso-nrel`)
 * Adam R. Jensen (:ghuser:`AdamRJensen`)
+* Johann Loux (:ghuser:`JoLo90`)

pvlib/forecast.py

@@ -512,10 +512,10 @@ def cloud_cover_to_irradiance_clearsky_scaling(self, cloud_cover,
     def cloud_cover_to_transmittance_linear(self, cloud_cover, offset=0.75,
                                             **kwargs):
         """
-        Convert cloud cover to atmospheric transmittance using a linear
-        model.
+        Convert cloud cover (percentage) to atmospheric transmittance
+        using a linear model.
 
-        0% cloud cover returns offset.
+        0% cloud cover returns "offset".
 
         100% cloud cover returns 0.
 
@@ -524,14 +524,15 @@ def cloud_cover_to_transmittance_linear(self, cloud_cover, offset=0.75,
         cloud_cover : numeric
             Cloud cover in %.
         offset : numeric, default 0.75
-            Determines the maximum transmittance.
+            Determines the maximum transmittance. [unitless]
         kwargs
             Not used.
 
         Returns
         -------
-        ghi : numeric
-            Estimated GHI.
+        transmittance : numeric
+            The fraction of extraterrestrial irradiance that reaches
+            the ground. [unitless]
         """
         transmittance = ((100.0 - cloud_cover) / 100.0) * offset