docs: add link to PollSender (#3613)

There was no link to [`PollSender`] in [`mpsc::Sender`] docs which made it
less discoverable.

[`PollSender`]: https://docs.rs/tokio-util/0.6.4/tokio_util/sync/struct.PollSender.html
[`mpsc::Sender::send`]: https://docs.rs/tokio/1.3.0/tokio/sync/mpsc/struct.Sender.html

tokio/src/sync/mpsc/bounded.rs

@@ -16,6 +16,11 @@ use std::task::{Context, Poll};
 /// Send values to the associated `Receiver`.
 ///
 /// Instances are created by the [`channel`](channel) function.
+///
+/// To use the `Sender` in a poll function, you can use the [`PollSender`]
+/// utility.
+///
+/// [`PollSender`]: https://docs.rs/tokio-util/0.6/tokio_util/sync/struct.PollSender.html
 pub struct Sender<T> {
     chan: chan::Tx<T, Semaphore>,
 }