sync: polish and update API doc examples (#1398)

- Remove `poll_*` fns from some of the sync types.
- Move `AtomicWaker` and `Lock` to the root of the `sync` crate.

tokio-sync/tests/fuzz_semaphore.rs

@@ -9,8 +9,8 @@ mod semaphore;
 
 use crate::semaphore::*;
 
-use async_util::future::poll_fn;
 use futures_core::ready;
+use futures_util::future::poll_fn;
 use loom::futures::block_on;
 use loom::thread;
 use std::future::Future;