update aliasing rules section of the reference

Author: RalfJung

src/behavior-considered-undefined.md

@@ -26,8 +26,15 @@ code.
 * Evaluating a [dereference expression] (`*expr`) on a raw pointer that is
   [dangling] or unaligned, even in [place expression context]
   (e.g. `addr_of!(&*expr)`).
-* Breaking the [pointer aliasing rules]. `&mut T` and `&T` follow LLVM’s scoped
-  [noalias] model, except if the `&T` contains an [`UnsafeCell<U>`].
+* Breaking the [pointer aliasing rules]. `Box<T>`, `&mut T` and `&T` follow LLVM’s
+  scoped noalias model, except if the `&T` contains an [`UnsafeCell<U>`].
+  References must not be dangling while they are live. (The exact liveness
+  duration is not specified, but it is certainly upper-bounded by the syntactic
+  lifetime assigned by the borrow checker. When a reference is passed to a
+  function, it is live at least as long as that function call, again except if
+  the `&T` contains an [`UnsafeCell<U>`].) All this also applies when values of
+  these types are passed in a (nested) field of a compound type, but not behind
+  pointer indirections.
 * Mutating immutable data. All data inside a [`const`] item is immutable. Moreover, all
   data reached through a shared reference or data owned by an immutable binding
   is immutable, unless that data is contained within an [`UnsafeCell<U>`].