[Docs] Add note about archival

docs/user/40_export.rst

@@ -9,3 +9,6 @@ a collection of all the students' personal notes as well as all lesson contents.
 
 An alternative path to printing a class register is via the weekly view.
 There, the relevant group can be selected via the filter to print the class book.
+
+.. note::
+   This mechanism can be used for archival purposes required by law.