More docs.

shell/platform/embedder/embedder.h

@@ -459,6 +459,10 @@ typedef void (*FlutterTaskRunnerPostTaskCallback)(
     uint64_t /* target time nanos */,
     void* /* user data */);
 
+// An interface used by the Flutter engine to execute tasks at the target time
+// on a specified thread. There should be a 1-1 relationship between a thread
+// and a task runner. It is undefined behavior to run a task on a thread that is
+// not associated with its task runner.
 typedef struct {
   // The size of this struct. Must be sizeof(FlutterTaskRunnerDescription).
   size_t struct_size;
@@ -469,8 +473,9 @@ typedef struct {
   // This field is required.
   BoolCallback runs_task_on_current_thread_callback;
   // May be called from any thread. The given task should be executed by the
-  // embedder by calling FlutterEngineRunTask at the given target time. The
-  // system monotonic clock should be used for the target time.
+  // embedder on the thread associated with that task runner by calling
+  // FlutterEngineRunTask at the given target time. The system monotonic clock
+  // should be used for the target time.
   //
   // This field is required.
   FlutterTaskRunnerPostTaskCallback post_task_callback;