remove old files, add general text to glossary

Author: RDaxini

docs/sphinx/source/user_guide/glossary.rst

@@ -3,6 +3,8 @@
 Glossary
 ========
 
+There is a convention on consistent variable names throughout the library:
+
 .. glossary::
 
     airmass
@@ -179,3 +181,16 @@ Glossary
 
     wind_speed
         Wind speed
+
+
+For a definition and further explanation on the variables, common symbols and
+units refer to the following sources:
+
+* Explanation of Solar irradiation and solar geometry by `SoDa Service <http://www.soda-pro.com/home>`_
+  
+   * `Acronyms, Terminology and Units <https://www.soda-pro.com/help/general/acronyms-terminology-and-units>`_
+   * `Plane orientations and radiation components <https://www.soda-pro.com/help/general/plane-orientations-and-radiation-components>`_
+   * `Time references <https://www.soda-pro.com/help/general/time-references>`_
+
+.. note:: These further references might not use the same terminology as
+          *pvlib*. But the physical process referred to is the same.