Clarify Options::rate_limiter api (#9361)

Summary:
Context/Summary:
I believe we also rate-limit read rate using the rate limiter passed into db options, e.g, https://github.com/facebook/rocksdb/blob/6.27.fb/file/random_access_file_reader.cc#L159

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9361

Test Plan: Existing tests

Reviewed By: jay-zhuang

Differential Revision: D33420803

Pulled By: hx235

fbshipit-source-id: 0ef3c4d0aaacb9bee9a5d2caceddfc76588c8949

include/rocksdb/options.h

@@ -513,7 +513,7 @@ struct DBOptions {
   // Default: Env::Default()
   Env* env = Env::Default();
 
-  // Use to control write rate of flush and compaction. Flush has higher
+  // Use to control write/read rate of flush and compaction. Flush has higher
   // priority than compaction. Rate limiting is disabled if nullptr.
   // If rate limiter is enabled, bytes_per_sync is set to 1MB by default.
   // Default: nullptr