add clarification for numbers eg aod500

Author: RDaxini

docs/sphinx/source/user_guide/extras/nomenclature.rst

@@ -21,9 +21,13 @@ There is a convention on consistent variable names throughout the library:
         [unitless]
 
     aod
+    aod500
         aerosol optical depth. Measure of aerosols (e.g., smoke
         particles, desert dust) distributed within a column of air from the
-        instrument (Earth's surface) to the top of the atmosphere. [unitless]
+        instrument (Earth's surface) to the top of the atmosphere. The AOD
+        value indicates the level of extinction of sunlight in this column, and
+        when followed by a number (e.g. AOD500), indicates the extinction at
+        this wavelength (500nm). [unitless]
 
     aoi
         Angle of incidence. Angle between the surface normal vector and the