task: expand on cancellation of spawn_blocking (#4811)

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

@@ -102,11 +102,21 @@ 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`] that the task checks periodically.
+    /// Another approach is to have the thread primarily read or write from a channel,
+    /// and to exit when the channel closes; assuming the other side of the channel is dropped
+    /// when cancellation occurs, this will cause the blocking task 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.
+    /// are simply allowed to keep running after the method returns.  It is possible
+    /// for a blocking task to be cancelled if it has not yet started running, but this
+    /// is not guaranteed.
     ///
     /// Note that if you are using the single threaded runtime, this function will
     /// still spawn additional threads for blocking operations. The basic
@@ -140,6 +150,7 @@ cfg_rt! {
     /// [`thread::spawn`]: fn@std::thread::spawn
     /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
     /// [bridgesync]: https://tokio.rs/tokio/topics/bridging
+    /// [`AtomicBool`]: struct@std::sync::atomic::AtomicBool
     ///
     /// # Examples
     ///