Neaten bulkhead tests commentary

src/Polly.Specs/Helpers/Bulkhead/TraceableAction.cs

@@ -45,6 +45,11 @@ public void SignalStateChange()
             _statusChanged.Set();
         }
 
+        // Note re TaskCreationOptions.LongRunning: Testing the parallelization of the bulkhead policy efficiently requires the ability to start large numbers of parallel tasks in a short space of time.
+        // The ThreadPool's algorithm of only injecting extra threads (when necessary) at a rate of two-per-second however makes high-volume tests using the ThreadPool both slow and flaky.  In the days of PCL, further, ThreadPool.SetMinThreads(...) was not available to mitigate this.
+        // Using TaskCreationOptions.LongRunning allows us to force tasks to be started near-instantly on non-ThreadPool threads.
+        // Similarly, we use ConfigureAwait(true) when awaiting, to avoid continuations being scheduled onto a ThreadPool thread, which may only be injected too slowly in high-volume tests.
+
         public Task ExecuteOnBulkhead(BulkheadPolicy bulkhead)
         {
             return ExecuteThroughSyncBulkheadOuter(