Update comment on deprecation warning to point inernal users to use safer API (#12318)

Summary:

For Meta internal use there is UseNThreadsThreadPoolGuard API to allow users to use a subset of threads. Point users to this API.

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Reviewed By: swolchok

Differential Revision: D77973302

Author: kimishpatel

extension/threadpool/threadpool.h

@@ -42,8 +42,13 @@ class ThreadPool final {
    * is a private API, which will later be replaced by something that allows
    * creating of threadpool with requested size and use such a threadpool with
    * backend delegates, custom ops or optimized lib.
+   * For Meta internal use, there is
+   * executorch::extension::threadpool::UseNThreadsThreadPoolGuard API that
+   * provides a safer way to select a subset of threads, from threadpool, to run
+   * the model on.
    */
-  [[deprecated("This API is experimental and may change without notice.")]]
+  [[deprecated(
+      "This API is experimental and may change without notice. Consider using UseNThreadsThreadPoolGuard")]]
   bool _unsafe_reset_threadpool(uint32_t num_threads);
 
   /**