Update TaskScheduler docs to make it more obvious how a TaskScheduler should be put in place.

base/task_scheduler/task_scheduler.h

Index: base/task_scheduler/task_scheduler.h
diff --git a/base/task_scheduler/task_scheduler.h b/base/task_scheduler/task_scheduler.h
index 296e40bb8dffe3f8c35263b4f299130c9175b54a..8549689bf14baf0ab328c9d64c4eb87ef53650e9 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -108,7 +108,8 @@ class BASE_EXPORT TaskScheduler {
   // new TaskScheduler.
 
   // Creates and sets a task scheduler with one worker pool that can have up to
-  // |max_threads| threads. CHECKs on failure.
+  // |max_threads| threads. CHECKs on failure. For tests, prefer
+  // base::test::ScopedTaskScheduler (ensures isolation).
   static void CreateAndSetSimpleTaskScheduler(int max_threads);
 
   // Creates and sets a task scheduler with custom worker pools. CHECKs on
@@ -121,7 +122,8 @@ class BASE_EXPORT TaskScheduler {
           worker_pool_index_for_traits_callback);
 
   // Registers |task_scheduler| to handle tasks posted through the post_task.h
-  // API for this process.
+  // API for this process. For tests, prefer base::test::ScopedTaskScheduler
+  // (ensures isolation).
   static void SetInstance(std::unique_ptr<TaskScheduler> task_scheduler);
 
   // Retrieve the TaskScheduler set via CreateAndSetDefaultTaskScheduler() or