Add comment clarifying test

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: carbotaniuman

tests/compile-fail/ptr_int_guess.rs

@@ -6,6 +6,7 @@ fn main() {
 
     // TODO: switch this to addr() once we intrinsify it
     let x_usize: usize = unsafe { std::mem::transmute(x_ptr) };
+    // Cast back a pointer that did *not* get exposed.
     let ptr = x_usize as *const i32;
     assert_eq!(unsafe { *ptr }, 3); //~ ERROR Undefined Behavior: dereferencing pointer failed
 }