Auto merge of #29600 - Ryman:rustdoc_better_testnames, r=alexcrichton

Old doctest names
```bash
test sync::atomic::load_0 ... ok
test sync::atomic::load_0 ... ok
test sync::atomic::load_0 ... ok
test sync::atomic::load_0 ... ok
```

New doctest names
```bash
test sync::atomic::AtomicBool::load_0 ... ok
test sync::atomic::AtomicIsize::load_0 ... ok
test sync::atomic::AtomicPtr<T>::load_0 ... ok
test sync::atomic::AtomicUsize::load_0 ... ok
```