tcp: update API documentation

[carllerche]

No description provided.

[95th]

This and #1398 is causing Hyper build to fail with:

error[E0599]: no method named `poll_ref` found for type `tokio_sync::watch::Receiver<common::drain::Action>` in the current scope
   --> src/common/drain.rs:102:39
    |
102 |                     match me.watch.rx.poll_ref(cx) {
    |                                       ^^^^^^^^

error[E0624]: method `poll_accept` is private
   --> src/server/tcp.rs:109:48
    |
109 |             match Pin::new(&mut self.listener).poll_accept(cx) {

@carllerche I know you discussed on gitter about making poll_* private and exposing only async methods, but libraries that want to use lower level API and custom polling logic are breaking.