Minor improvements of debugging experience (#2093)

* Recognizable toString implementation in default dispatchers
* Fast-path for disabled debug probes
diff --git a/docs/flow.md b/docs/flow.md
index 9b33158..f0f570d 100644
--- a/docs/flow.md
+++ b/docs/flow.md
@@ -697,7 +697,7 @@
 ```text
 Exception in thread "main" java.lang.IllegalStateException: Flow invariant is violated:
 		Flow was collected in [CoroutineId(1), "coroutine#1":BlockingCoroutine{Active}@5511c7f8, BlockingEventLoop@2eac3323],
-		but emission happened in [CoroutineId(1), "coroutine#1":DispatchedCoroutine{Active}@2dae0000, DefaultDispatcher].
+		but emission happened in [CoroutineId(1), "coroutine#1":DispatchedCoroutine{Active}@2dae0000, Dispatchers.Default].
 		Please refer to 'flow' documentation or use 'flowOn' instead
 	at ...
 ```