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.

First, basically no real world case will spawn
a closure that takes no parameters, so
change the first example to pass an input value, mutate it, and
receive the result.

Let's link to the channel blocking APIs, and provide a small example
of that too.

This also mentions `SyncIoBridge` now.

tokio/src/task/blocking.rs

@@ -112,27 +112,77 @@ 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 [`mpsc channel`] has `blocking_send` and
+    /// `blocking_recv` methods for use in non-async code such as the thread created
+    /// by `spawn_blocking`.
+    ///
+    /// Another option is [`SyncIoBridge`] for cases where the synchronous context
+    /// is operating on byte streams.  For example, you might use an asynchronous
+    /// HTTP client such as [hyper] to fetch data, but perform complex parsing
+    /// of the payload body using a library written for synchronous I/O.
+    ///
     /// [`Builder`]: struct@crate::runtime::Builder
     /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
     /// [rayon]: https://docs.rs/rayon
+    /// [`mpsc channel`]: crate::sync::mpsc
+    /// [`SyncIoBridge`]: https://docs.rs/tokio-util/0.6/tokio_util/io/struct.SyncIoBridge.html
+    /// [hyper]: https://docs.rs/hyper
     /// [`thread::spawn`]: fn@std::thread::spawn
     /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
     ///
     /// # Examples
     ///
+    /// Pass an input value and receive result of computation:
+    ///
     /// ```
     /// use tokio::task;
     ///
     /// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
+    /// // Initial input
+    /// let mut v = "Hello, ".to_string();
     /// let res = task::spawn_blocking(move || {
-    ///     // do some compute-heavy work or call synchronous code
-    ///     "done computing"
+    ///     // Stand-in for compute-heavy work or using synchronous APIs
+    ///     v.push_str("world");
+    ///     // Pass ownership of the value back to the asynchronous context
+    ///     v
     /// }).await?;
     ///
-    /// assert_eq!(res, "done computing");
+    /// // `res` is the value returned from the thread
+    /// assert_eq!(res.as_str(), "Hello, world");
     /// # Ok(())
     /// # }
     /// ```
+    ///
+    /// Use a channel:
+    ///
+    /// ```
+    /// 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, 95);
+    /// worker.await.unwrap();
+    /// # }
+    /// ```
     #[cfg_attr(tokio_track_caller, track_caller)]
     pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
     where