Clarify Options::rate_limiter api doc for #9607 Rate-limit automatic WAL flush after each user write (#9745)

Summary:
As title for https://github.com/facebook/rocksdb/pull/9607

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9745

Test Plan: No code change

Reviewed By: ajkr

Differential Revision: D35096901

Pulled By: hx235

fbshipit-source-id: 6bd3671baecfdc04579b0a81a957bfaa7bed81e1

include/rocksdb/options.h

@@ -507,6 +507,9 @@ struct DBOptions {
   // - Reads associated with a `ReadOptions` can be charged at
   //   `ReadOptions::rate_limiter_priority` (see that option's API doc for usage
   //   and limitations).
+  // - Writes associated with a `WriteOptions` can be charged at
+  //   `WriteOptions::rate_limiter_priority` (see that option's API doc for
+  //   usage and limitations).
   //
   // Rate limiting is disabled if nullptr. If rate limiter is enabled,
   // bytes_per_sync is set to 1MB by default.