on [`Relaxed`], and
using [`Release`] makes the load part [`Relaxed`].

This function may generate more efficient code than `fetch_sub` on some platforms.

- MSP430: `sub` instead of disabling interrupts ({8,16}-bit atomics)

# Examples

```
use portable_atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(20);
foo.sub(10, Ordering::SeqCst);
assert_eq!(foo.load(Ordering::SeqCst), 10);
```