Add warning about reference validity

Author: gnzlbg

reference/src/layout/pointers.md

@@ -32,8 +32,12 @@ and are at least one word.
 The layouts of `&T`, `&mut T` and `*T` are the same.
 
 If `T` is sized, references and pointers to `T` have a size and alignment of one
-word and have therefore the same layout as C pointers - they can be used in C FFI
-where pointers are expected.
+word and have therefore the same layout as C pointers.
+
+> **warning**: while the layout of references and pointers is compatible with
+> the layout of C pointers, references come with a _validity_ invariant that
+> does not allow them to be used when they could be `NULL`, unaligned, dangling,
+> or, in the case of `&mut T`, aliasing.
 
 We do not make any guarantees about the layout of
 multi-trait objects `&(dyn T + U)` or references to other dynamically sized types,