Clarify docstring descriptions of cross-axis slope (#1530)

* Description for tracking.calc_cross_axis_tilt
The previous description for the signs of cross axis tilt mentioned slope azimuth as east for both positive and negative. Corrected it to indicate that sign of cross axis tilt is positive when axis azimuth is south and slope is towards west.

* up to down

* Update docs/sphinx/source/whatsnew/v0.9.3.rst

Co-authored-by: Kevin Anderson <kevin.anderson@nrel.gov>

* Update docs/sphinx/source/whatsnew/v0.9.3.rst

Co-authored-by: Kevin Anderson <kevin.anderson@nrel.gov>

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
Co-authored-by: Kevin Anderson <kevin.anderson@nrel.gov>

docs/sphinx/source/whatsnew/v0.9.3.rst

@@ -35,6 +35,7 @@ Testing
 
 Documentation
 ~~~~~~~~~~~~~
+* Clarified description of cross-axis slope in :py:mod:`pvlib.tracking` (:pull:`1530`)
 
 
 Benchmarking
@@ -57,4 +58,5 @@ Contributors
 * Mark Campanelli (:ghuser:`markcampanelli`)
 * Adam R. Jensen (:ghuser:`AdamRJensen`)
 * Abhishek Parikh (:ghuser:`abhisheksparikh`)
-* Taos Transue (:ghuser:`reepoi`)
+* Taos Transue (:ghuser:`reepoi`)
+* (:ghuser:`chiragpachori`)

pvlib/tracking.py

@@ -54,7 +54,7 @@ class SingleAxisTracker(PVSystem):
         using a right-handed convention. For example, trackers with axis
         azimuth of 180 degrees (heading south) will have a negative cross-axis
         tilt if the tracker axes plane slopes down to the east and positive
-        cross-axis tilt if the tracker axes plane slopes up to the east. Use
+        cross-axis tilt if the tracker axes plane slopes down to the west. Use
         :func:`~pvlib.tracking.calc_cross_axis_tilt` to calculate
         `cross_axis_tilt`. [degrees]
 
@@ -339,7 +339,7 @@ def singleaxis(apparent_zenith, apparent_azimuth,
         using a right-handed convention. For example, trackers with axis
         azimuth of 180 degrees (heading south) will have a negative cross-axis
         tilt if the tracker axes plane slopes down to the east and positive
-        cross-axis tilt if the tracker axes plane slopes up to the east. Use
+        cross-axis tilt if the tracker axes plane slopes down to the west. Use
         :func:`~pvlib.tracking.calc_cross_axis_tilt` to calculate
         `cross_axis_tilt`. [degrees]
 
@@ -641,7 +641,7 @@ def calc_cross_axis_tilt(
     specified using a right-handed convention. For example, trackers with axis
     azimuth of 180 degrees (heading south) will have a negative cross-axis tilt
     if the tracker axes plane slopes down to the east and positive cross-axis
-    tilt if the tracker axes plane slopes up to the east.
+    tilt if the tracker axes plane slopes down to the west.
 
     Parameters
     ----------