Leave validity to the table

src/expressions/operator-expr.md

@@ -480,15 +480,12 @@ assert_eq!(values[1], 3);
 
 #### Pointer-to-pointer cast
 
-`*const T` / `*mut T` can be cast to `*const U` / `*mut U` under the following
-conditions:
+`*const T` / `*mut T` can be cast to `*const U` / `*mut U` with the following behavior:
 
-- If both `T` and `U` are sized, the cast produces a value identical to the
-  original pointer.
-- If both `T` and `U` are unsized, the cast produces a value identical to the
-  original pointer if `T` and `U` are compatible unsized types.
 - If `T` is unsized and `U` is sized, the cast discards all metadata that
   completes the wide pointer `T` and produces a thin base pointer `U`.
+- Otherwise a cast from `T` to `U` produces a value identical to the original
+  pointer.
 
 ## Assignment expressions