Add comment for new_memory_used parameter in CacheReservationManager::UpdateCacheReservation (#8895)

Summary:
Context/Summary: this PR is to clarify what the parameter new_memory_used is in CacheReservationManager::UpdateCacheReservation

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8895

Test Plan:
- Passing existing test
- Make format

Reviewed By: jay-zhuang

Differential Revision: D30844814

Pulled By: hx235

fbshipit-source-id: 3177f7abf5668ea9e73818ceaa355566f03acabc

cache/cache_reservation_manager.h

@@ -67,8 +67,9 @@ class CacheReservationManager {
   // [cache_allocated_size_ * 3/4, cache_allocated_size) when delayed_decrease
   // is set true.
   //
-  // On inserting dummy entries, it returns Status::OK() if all dummy entry
+  // @param new_memory_used The number of bytes used by new memory
-  // insertions succeed. Otherwise, it returns the first non-ok status;
+  // @return On inserting dummy entries, it returns Status::OK() if all dummy
+  // entry insertions succeed. Otherwise, it returns the first non-ok status;
   // On releasing dummy entries, it always returns Status::OK().
   // On keeping dummy entries the same, it always returns Status::OK().
   Status UpdateCacheReservation(std::size_t new_memory_used);