subtyping: clarify the concreteness of 'a in example

This matters because if 'a were free to choose, then the argument would
be trivial.  But in reality the argument also holds if 'a was already
chosen, which is what makes lifetimes special.

src/subtyping.md

@@ -11,10 +11,11 @@ confusion, because it seems intuitively backwards to many: the bigger scope is a
 *subtype* of the smaller scope.
 
 This does in fact make sense, though. The intuitive reason for this is that if
-you expect an `&'a u8`, then it's totally fine for me to hand you an `&'static
-u8`, in the same way that if you expect an Animal in Java, it's totally fine for
-me to hand you a Cat. Cats are just Animals *and more*, just as `'static` is
-just `'a` *and more*.
+you expect an `&'a u8` (for some concrete `'a` that you have already chosen),
+then it's totally fine for me to hand you an `&'static u8` even if `'static !=
+'a`, in the same way that if you expect an Animal in Java, it's totally fine
+for me to hand you a Cat. Cats are just Animals *and more*, just as `'static`
+is just `'a` *and more*.
 
 (Note, the subtyping relationship and typed-ness of lifetimes is a fairly
 arbitrary construct that some disagree with. However it simplifies our analysis