Note that class variables as postulates are fine

Attempt to resolve a comment from Scott Fenton (@sctfn).

Signed-off-by: David A. Wheeler <dwheeler@dwheeler.com>

set.mm

@@ -431162,9 +431162,12 @@ be applied for new theorems (formerly, the class and wff variables
        <p>You should read any statement starting with ` |- `
        and having one or more unbound set variable(s) as universally
        quantified over those
-       variables. So when you see ` |- 0 < k ` that's saying " ` 0 < k ` is
-       derivable in the empty context" which is a strong statement,
-       equivalent to ` A. k 0 < k ` which without further context is false.
+       variables. So if you state ` |- 0 < k ` , say as a postulate,
+       that's saying " ` 0 < k ` is derivable in the empty context"
+       which is a strong statement,
+       equivalent to ` A. k 0 < k ` , which without further context is false.
+       The same is not true for class variables (e.g., ` 0 < K ` is typically
+       safe to postulate).
 
        <p><b>Additional rules for definitions</b>