Mention pointers to ZSTs

Author: Manishearth

src/advanced_unsafety/invalid_values.md

@@ -34,6 +34,8 @@ It's also currently invalid for `Vec<T>` to have a null pointer for its buffer!
 
 There are a lot of other reasons that a pointer type may not be valid, but these are the ones where the bit pattern is statically known to be invalid regardless of context. We'll be covering these in more depth in other chapters (@@note: where?), but, for example, all of these pointers must not only be non-null, they must also point to an actual valid instance of that type at all times (except `Vec<T>`, which is allowed to refer to invalid-but-aligned-and-non-null memory when it is empty)
 
+@@ Figure out details for where ZST pointers can point, see https://doc.rust-lang.org/std/ptr/index.html#safety
+
 #### "shallow" vs "deep" validity