Fix F.27 example, closes #2171

CppCoreGuidelines.md

@@ -3605,6 +3605,7 @@ Using `std::shared_ptr` is the standard way to represent shared ownership. That
 
 ##### Example
 
+    {
         shared_ptr<const Image> im { read_image(somewhere) };
 
         std::thread t0 {shade, args0, top_left, im};
@@ -3612,8 +3613,11 @@ Using `std::shared_ptr` is the standard way to represent shared ownership. That
         std::thread t2 {shade, args2, bottom_left, im};
         std::thread t3 {shade, args3, bottom_right, im};
 
-    // detach threads
-    // last thread to finish deletes the image
+        // detaching threads requires extra care (e.g., to join
+        // before main ends), but even if we do detach t0..3 here ...
+    }
+    // ... shared_ptr ensures that eventually the last thread to
+    //     finish safely deletes the image
 
 ##### Note