spawn_blocking: Expand on cancellation a bit

I was looking at these docs again and I was thinking the
"cannot be cancelled" is misleading; let's talk about approaches
to do so.

tokio/src/task/blocking.rs

@@ -102,8 +102,17 @@ cfg_rt! {
     /// their own. If you want to spawn an ordinary thread, you should use
     /// [`thread::spawn`] instead.
     ///
-    /// Closures spawned using `spawn_blocking` cannot be cancelled. When you shut
-    /// down the executor, it will wait indefinitely for all blocking operations to
+    /// Closures spawned using `spawn_blocking` cannot be cancelled abruptly; there
+    /// is no standard low level API to cause a thread to stop running.  However,
+    /// a useful pattern is to pass some form of "cancellation token" into
+    /// the thread.  This could be an [`AtomicBool`], and inner loops in the
+    /// thread check the boolean periodically.  Another approach is to
+    /// have the thread primarily read from a channel, and to exit when
+    /// the channel closes; assuming the sender side of the channel is dropped
+    /// when cancellation occurs, this will cause the receiver thread to exit
+    /// soon after as well.
+    ///
+    /// When you shut down the executor, it will wait indefinitely for all blocking operations to
     /// finish. You can use [`shutdown_timeout`] to stop waiting for them after a
     /// certain timeout. Be aware that this will still not cancel the tasks — they
     /// are simply allowed to keep running after the method returns.
@@ -140,6 +149,7 @@ cfg_rt! {
     /// [`thread::spawn`]: fn@std::thread::spawn
     /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
     /// [bridgesync]: https://tokio.rs/tokio/topics/bridging
+    /// [`AtomicBool`]: https://doc.rust-lang.org/core/sync/atomic/struct.AtomicBool.html
     ///
     /// # Examples
     ///