Fix memory64 feature to pass (rep i64) when lowering borrow

Author: lukewagner

design/mvp/Binary.md

@@ -292,8 +292,8 @@ Notes:
   `none` case of an optional immediate.)
 * 🔧 for fixed-sized lists the length of the list must be larger than 0 to pass
   validation.
-* Validation requires that, for every `defvaltype` `t`, `elem_size(t, 'i64')` is
-  less than 2<sup>28</sup>, as defined by the
+* Validation requires that, for every `defvaltype` `t`, the worst-case in-memory
+  element size is less than 2<sup>28</sup>, as defined by `elem_size` in the
   [Canonical ABI](CanonicalABI.md#element-size).