reference: note the existence of UnsafeCell

Author: thestinger

src/doc/reference.md

@@ -1152,7 +1152,7 @@ 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