task: Add more tips + links to spawn_blocking docs

I'm working on some code which heavily uses `spawn_blocking`
to run synchronous code, and it took me a while to find and understand
the relevant APIs and patterns here.  Let's link to the channel
blocking APIs, and provide a small example.

I plan to mention #4146 here
too once that merges.

tokio/src/task/blocking.rs

@@ -111,9 +111,22 @@ cfg_rt! {
     /// still spawn additional threads for blocking operations. The basic
     /// scheduler's single thread is only used for asynchronous code.
     ///
+    /// # Related APIs and patterns for bridging asynchronous and blocking code
+    ///
+    /// In simple cases, it is sufficient to have the closure accept input
+    /// parameters at creation time and return a single value (or struct/tuple, etc.).
+    ///
+    /// For more complex situations in which it is desirable to stream data to or from
+    /// the synchronous context, the [`channel APIs`] have [`crate::sync::mpsc::Sender::blocking_send`] and
+    /// [`crate::sync::mpsc::Receiver::blocking_recv`] that support being called
+    /// from the spawned thread's blocking context.  You may also find it useful
+    /// to wrap the channels in e.g. [`ReceiverStream`].
+    ///
     /// [`Builder`]: struct@crate::runtime::Builder
     /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
     /// [rayon]: https://docs.rs/rayon
+    /// [`channel APIs`]: crate::sync::mpsc
+    /// [`ReceiverStream`]: https://docs.rs/tokio-stream/0.1/tokio_stream/wrappers/struct.ReceiverStream.html
     /// [`thread::spawn`]: fn@std::thread::spawn
     /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
     ///
@@ -132,6 +145,29 @@ cfg_rt! {
     /// # Ok(())
     /// # }
     /// ```
+    ///
+    /// ```
+    /// use tokio::task;
+    /// use tokio::sync::mpsc;
+    ///
+    /// # async fn docs() {
+    /// let (tx, mut rx) = mpsc::channel(2);
+    /// let start = 5;
+    /// let worker = task::spawn_blocking(move || {
+    ///     for x in 0..10 {
+    ///         // Stand in for complex computation
+    ///         tx.blocking_send(start + x).unwrap();
+    ///     }
+    /// });
+    ///
+    /// let mut acc = 0;
+    /// while let Some(v) = rx.recv().await {
+    ///     acc += v;
+    /// }
+    /// assert_eq!(acc, 45);
+    /// worker.await.unwrap();
+    /// # }
+    /// ```
     #[cfg_attr(tokio_track_caller, track_caller)]
     pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
     where