make it more clear that the uninit-mem clause is type-driven

Author: RalfJung

src/behavior-considered-undefined.md

@@ -43,8 +43,8 @@ code.
   * A null `fn` pointer.
   * A value in a `char` which is a surrogate or above `char::MAX`.
   * A `!` (all values are invalid for this type).
-  * [Uninitialized memory][undef] in the value of an integer (`i*`/`u*`),
-    floating point value (`f*`), or raw pointer.
+  * An integer (`i*`/`u*`), floating point value (`f*`), or raw pointer obtained
+    from [uninitialized memory][undef].
   * A reference or `Box<T>` that is dangling, unaligned, or points to an invalid value.
   * Invalid metadata in a wide reference, `Box<T>`, or raw pointer:
     * `dyn Trait` metadata is invalid if it is not a pointer to a vtable for