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..da2cb79504e9c2472758abcfce4f5ddd51246686 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -108,25 +108,36 @@ 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
   // failure. |worker_pool_params_vector| describes the worker pools to create.
   // |worker_pool_index_for_traits_callback| returns the index in |worker_pools|
-  // of the worker pool in which a task with given traits should run.
+  // of the worker pool in which a task with given traits should run. For tests,
+  // prefer base::test::ScopedTaskScheduler (ensures isolation).
   static void CreateAndSetDefaultTaskScheduler(
       const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
       const WorkerPoolIndexForTraitsCallback&
           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
-  // SetInstance(). This should be used very rarely; most users of TaskScheduler
-  // should use the post_task.h API.
+  // Retrieve the TaskScheduler set via SetInstance() or
+  // CreateAndSet(Simple|Default)TaskScheduler(). This should be used very
+  // rarely; most users of TaskScheduler should use the post_task.h API. In
+  // particular, refrain from doing
+  //   if (!TaskScheduler::GetInstance()) {
+  //     TaskScheduler::SetInstance(...);
+  //     base::PostTask(...);
+  //   }
+  // instead make sure to SetInstance() early in one determinstic place in the
+  // process' initialization phase.
+  // In doubt, consult with //base/task_scheduler/OWNERS.
   static TaskScheduler* GetInstance();
 
  private: