Expand docstring for aoi_projection. (#648)

Author: adriesse

pvlib/irradiance.py

@@ -154,8 +154,11 @@ def to_doy(x): return x                                 # noqa: E306
 
 def aoi_projection(surface_tilt, surface_azimuth, solar_zenith, solar_azimuth):
     """
-    Calculates the dot product of the solar vector and the surface
-    normal.
+    Calculates the dot product of the sun position unit vector and the surface
+    normal unit vector; in other words, the cosine of the angle of incidence.
+
+    Usage note: When the sun is behind the surface the value returned is
+    negative.  For many uses negative values must be set to zero.
 
     Input all angles in degrees.