Finish doc of ScheduledTask.serialized_task (#7977)

I forgot to commit my clarification before merging.

proto/scheduler.proto

@@ -337,6 +337,7 @@ message ScheduleTaskRequest {
 
   // Serialized build.bazel.remote.execution.v2.ExecutionTask. That proto
   // depends on this one, so using the full type would create a build cycle.
+  // We could move ExecutionTask to this file to fix this.
   bytes serialized_task = 3;
 }