improve comments for statistics.h

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5351

Differential Revision: D15496346

Pulled By: miasantreble

fbshipit-source-id: eeb619e6bd8616003ba35b0cd4bb8050e6a8cb4d

include/rocksdb/statistics.h

@@ -447,6 +447,10 @@ struct HistogramData {
   double min = 0.0;
 };
 
+// StatsLevel can be used to reduce statistics overhead by skipping certain
+// types of stats in the stats collection process.
+// Usage:
+//   options.statistics->set_stats_level(StatsLevel::kExceptTimeForMutex);
 enum StatsLevel : uint8_t {
   // Disable timer stats, and skip histogram stats
   kExceptHistogramOrTimers,
@@ -464,7 +468,15 @@ enum StatsLevel : uint8_t {
   kAll,
 };
 
-// Analyze the performance of a db
+// Analyze the performance of a db by providing cumulative stats over time.
+// Usage:
+//  Options options;
+//  options.statistics = rocksdb::CreateDBStatistics();
+//  Status s = DB::Open(options, kDBPath, &db);
+//  ...
+//  options.statistics->getTickerCount(NUMBER_BLOCK_COMPRESSED);
+//  HistogramData hist;
+//  options.statistics->histogramData(FLUSH_TIME, &hist);
 class Statistics {
  public:
   virtual ~Statistics() {}