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_mpsc.rs

@@ -18,8 +18,7 @@ mod mpsc;
 #[allow(warnings)]
 mod semaphore;
 
-// use futures::{future::poll_fn, Stream};
-use async_util::future::poll_fn;
+use futures_util::future::poll_fn;
 use loom::futures::block_on;
 use loom::thread;