Make note of noalias in unsafe reference section

Fixes #19733

src/doc/reference.md

@@ -1231,12 +1231,15 @@ the guarantee that these issues are never caused by safe code.
 
 * Data races
 * Dereferencing a null/dangling raw pointer
-* Mutating an immutable value/reference without `UnsafeCell`
 * Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values)
   (uninitialized) memory
 * Breaking the [pointer aliasing
   rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules)
   with raw pointers (a subset of the rules used by C)
+* `&mut` and `&` follow LLVM’s scoped [noalias] model, except if the `&T`
+  contains an `UnsafeCell<U>`. Unsafe code must not violate these aliasing
+  guarantees.
+* Mutating an immutable value/reference without `UnsafeCell<U>`
 * Invoking undefined behavior via compiler intrinsics:
   * Indexing outside of the bounds of an object with `std::ptr::offset`
     (`offset` intrinsic), with
@@ -1253,6 +1256,8 @@ the guarantee that these issues are never caused by safe code.
   code. Rust's failure system is not compatible with exception handling in
   other languages. Unwinding must be caught and handled at FFI boundaries.
 
+[noalias]: http://llvm.org/docs/LangRef.html#noalias
+
 ##### Behaviour not considered unsafe
 
 This is a list of behaviour not considered *unsafe* in Rust terms, but that may