Fix minor code listing issues

Ignoring the value returned by addTaskUnlessCancelled(...) produces a
warning.  This version uses the value to stop early after cancellation.

Explicit comparison with true/false doesn't appear anywhere else in TSPL
(except for some hidden assertions in old swifttest blocks).  Use the
Boolean value on its own to match style.

Author: amartini51

TSPL.docc/LanguageGuide/Concurrency.md

@@ -712,10 +712,11 @@ like closing network connections and deleting temporary files.
 let photos = await withTaskGroup(of: Optional<Data>.self) { group in
     let photoNames = await listPhotos(inGallery: "Summer Vacation")
     for name in photoNames {
-        group.addTaskUnlessCancelled {
-            guard Task.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`.