Try to specify what the base pointer is

Author: udoprog

src/expressions/operator-expr.md

@@ -483,7 +483,8 @@ assert_eq!(values[1], 3);
 `*const T` / `*mut T` can be cast to `*const U` / `*mut U` with the following behavior:
 
 - 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`.
+  completes the wide pointer `T` and produces a thin pointer `U` to the
+  value of the unsized pointer.
 - Otherwise a cast from `T` to `U` produces a value identical to the original
   pointer.