elaborate on slice wide pointer metadata

Author: RalfJung

src/behavior-considered-undefined.md

@@ -72,11 +72,14 @@ Please read the [Rustonomicon] before writing unsafe code.
   * An integer (`i*`/`u*`), floating point value (`f*`), or raw pointer obtained
     from [uninitialized memory][undef], or uninitialized memory in a `str`.
   * A reference or `Box<T>` that is [dangling], misaligned, or points to an invalid value.
-  * Invalid metadata in a wide reference, `Box<T>`, or raw pointer:
+  * Invalid metadata in a wide reference, `Box<T>`, or raw pointer. The requirement
+    for the metadata is determined by the type of the unsized tail:
     * `dyn Trait` metadata is invalid if it is not a pointer to a vtable for
       `Trait` that matches the actual dynamic trait the pointer or reference points to.
     * Slice metadata is invalid if the length is not a valid `usize`
       (i.e., it must not be read from uninitialized memory).
+      Furthermore, for wide references and `Box<T>`, slice metadata is invalid
+      if it makes the total size of the type bigger than `isize::MAX`.
   * Invalid values for a type with a custom definition of invalid values.
     In the standard library, this affects [`NonNull<T>`] and [`NonZero*`].