Fix an inconsistency about whether 'cls' is preferred or not.

pep-0008.txt

@@ -573,8 +573,10 @@ Naming Conventions
 
       - If your public attribute name collides with a reserved keyword, append
         a single trailing underscore to your attribute name.  This is
-        preferable to an abbreviation or corrupted spelling.  E.g. "class_"
-        is preferable to "cls" or "klass".
+        preferable to an abbreviation or corrupted spelling.  (However,
+        notwithstanding this rule, 'cls' is the preferred spelling for any
+        variable or argument which is known to be a class, especially the
+        first argument to a class method.)
 
         Note 1: See the argument name recommendation above for class methods.