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-threadpool/src/shutdown.rs

@@ -1,7 +1,7 @@
 use crate::task::Task;
 use crate::worker;
 
-use tokio_sync::task::AtomicWaker;
+use tokio_sync::AtomicWaker;
 
 use crossbeam_deque::Injector;
 use std::future::Future;