aoi definition in variables_style_rules.csv (#2247)

* Update variables_style_rules.csv

* update definition

Co-Authored-By: Cliff Hansen <5393711+cwhanse@users.noreply.github.com>
Co-Authored-By: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>

* Update pvlib/data/variables_style_rules.csv

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

---------

Co-authored-by: Cliff Hansen <5393711+cwhanse@users.noreply.github.com>
Co-authored-by: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>
Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>

pvlib/data/variables_style_rules.csv

@@ -11,7 +11,7 @@ ghi;global horizontal irradiance
 ghi_extra;horizontal irradiance at top of atmosphere (extraterrestrial)
 gri;ground-reflected irradiance
 ape;average photon energy
-aoi;angle of incidence between :math:`90\deg` and :math:`90\deg`
+aoi;angle of incidence. Angle between the surface normal vector and the vector pointing towards the sun's center
 aoi_projection;cos(aoi)
 airmass;airmass
 airmass_relative;relative airmass