fix and expand what we say about ZST

Author: RalfJung

src/exotic-sizes.md

@@ -111,10 +111,15 @@ support values.
 
 Safe code need not worry about ZSTs, but *unsafe* code must be careful about the
 consequence of types with no size. In particular, pointer offsets are no-ops,
-and standard allocators may return `null` when a zero-sized allocation is
-requested, which is indistinguishable from the out of memory result.
+and allocators typically [require a non-zero size][alloc].
 
+Note that references to ZSTs (including empty slices), just like all other
+references, must be non-null and suitably aligned. Dereferencing a null or
+unaligned pointer to a ZST is [undefined behavior][ub], just like for any other
+type.
 
+[alloc]: https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html#tymethod.alloc
+[ub]: what-unsafe-does.html