reference: note the existence of UnsafeCell
@@ -1153,7 +1153,7 @@ exposing an API making it possible for it to occur in safe code.
* Data races
* Dereferencing a null/dangling raw pointer
-* Mutating an immutable value/reference
+* Mutating an immutable value/reference without `UnsafeCell`
* Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values)
(uninitialized) memory
* Breaking the [pointer aliasing
