[continuous profiler] code comment in test application for context tracking (#3235)

test/test-applications/integrations/TestApplication.ContinuousProfiler.ContextTracking/Program.cs

@@ -11,7 +11,10 @@ internal class Program
 
     public static async Task Main(string[] args)
     {
+        // Start an activity that remains active until async operation completes,
+        // and verify that trace context flows properly between threads that carry out parts of the async operation.
         using var activity = Source.StartActivity();
+
         await DoSomethingAsync();
     }