Update get_ground_diffuse description (#1883)

* Update documentation

* Add Cliff's statement

* undo edits to v0.10.3 whatsnew

* whatsnew

* Update pvlib/irradiance.py

---------

Co-authored-by: Cliff Hansen <[email protected]>

Author: AdamRJensen

docs/sphinx/source/whatsnew/v0.10.4.rst

@@ -21,6 +21,7 @@ Testing
 Documentation
 ~~~~~~~~~~~~~
 * Fixed broken URLs in various docstrings. (:pull:`1957`)
+* Clarified documentation for :py:func:`~pvlib.irradiance.get_ground_diffuse`. (:pull:`1883`)
 
 Requirements
 ~~~~~~~~~~~~
@@ -29,3 +30,4 @@ Requirements
 Contributors
 ~~~~~~~~~~~~
 * :ghuser:`matsuobasho`
+* Adam R. Jensen (:ghuser:`AdamRJensen`)

pvlib/irradiance.py

@@ -553,7 +553,9 @@ def poa_components(aoi, dni, poa_sky_diffuse, poa_ground_diffuse):
 def get_ground_diffuse(surface_tilt, ghi, albedo=.25, surface_type=None):
     '''
     Estimate diffuse irradiance from ground reflections given
-    irradiance, albedo, and surface tilt.
+    irradiance, albedo, and surface tilt. The ground is assumed to
+    be horizontal, flat and Lambertian, and the reflected irradiance
+    is isotropic.
 
     Function to determine the portion of irradiance on a tilted surface
     due to ground reflections. Any of the inputs may be DataFrames or