Add note about use of force_ptr in futex implementation.

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

src/shims/posix/linux/sync.rs

@@ -30,6 +30,8 @@ pub fn futex<'tcx>(
 
  // The raw pointer value is used to identify the mutex.
  // Not all mutex operations actually read from this address or even require this address to exist.
+ // This will make FUTEX_WAKE fail on an integer cast to a pointer. But FUTEX_WAIT on
+ // such a pointer can never work anyway, so that seems fine.
  let futex_ptr = this.force_ptr(addr.to_scalar()?)?;
 
  let thread = this.get_active_thread();