Fixes task cancellation sample code (#279)

The code listing tried to access the `isCancelled` property without
specifying `Task.isCancelled`. That would work in the scope of a task,
because there's an instance property with that name. But it doesn't work
in the context of calling `addTaskUnlessCancelled(priority:operation:)`
to adding a child task to a task group -- in that case, you need to use
the type property.

Author: amartini51

TSPL.docc/LanguageGuide/Concurrency.md

@@ -695,8 +695,8 @@ To let the user stop this work,
 without waiting for all of the tasks to complete,
 the tasks need check for cancellation and stop running if they are canceled.
 There are two ways a task can do this:
-by calling the [`Task.checkCancellation()`][] method,
-or by reading the [`Task.isCancelled`][] property.
+by calling the [`Task.checkCancellation()`][] type method,
+or by reading the [`Task.isCancelled`][`Task.isCancelled` type] type property.
 Calling `checkCancellation()` throws an error if the task is canceled;
 a throwing task can propagate the error out of the task,
 stopping all of the task's work.
@@ -706,16 +706,17 @@ which lets you perform clean-up work as part of stopping the task,
 like closing network connections and deleting temporary files.
 
 [`Task.checkCancellation()`]: https://developer.apple.com/documentation/swift/task/3814826-checkcancellation
-[`Task.isCancelled`]: https://developer.apple.com/documentation/swift/task/3814832-iscancelled
+[`Task.isCancelled` type]: https://developer.apple.com/documentation/swift/task/iscancelled-swift.type.property
 
 ```
 let photos = await withTaskGroup(of: Optional<Data>.self) { group in
     let photoNames = await listPhotos(inGallery: "Summer Vacation")
     for name in photoNames {
-        group.addTaskUnlessCancelled {
-            guard isCancelled == false else { return nil }
+        let added = group.addTaskUnlessCancelled {
+            guard !Task.isCancelled else { return nil }
             return await downloadPhoto(named: name)
         }
+        guard added else { break }
     }
 
     var results: [Data] = []
@@ -732,6 +733,11 @@ The code above makes several changes from the previous version:
   [`TaskGroup.addTaskUnlessCancelled(priority:operation:)`][] method,
   to avoid starting new work after cancellation.
 
+- After each call to `addTaskUnlessCancelled(priority:operation:)`,
+  the code confirms that the new child task was added.
+  If the group is canceled, the value of `added` is `false` ---
+  in that case, the code stops trying to download additional photos.
+
 - Each task checks for cancellation
   before starting to download the photo.
   If it has been canceled, the task returns `nil`.
@@ -745,6 +751,13 @@ The code above makes several changes from the previous version:
 
 [`TaskGroup.addTaskUnlessCancelled(priority:operation:)`]: https://developer.apple.com/documentation/swift/taskgroup/addtaskunlesscancelled(priority:operation:)
 
+> Note:
+> To check whether a task has been canceled from outside that task,
+> use the [`Task.isCancelled`][`Task.isCancelled` instance] instance property
+> instead of the type property.
+
+[`Task.isCancelled` instance]: https://developer.apple.com/documentation/swift/task/iscancelled-swift.property
+
 For work that needs immediate notification of cancellation,
 use the [`Task.withTaskCancellationHandler(operation:onCancel:)`][] method.
 For example: