address review comment

Author: cbi42

include/rocksdb/utilities/write_batch_with_index.h

@@ -88,6 +88,7 @@ class WBWIIterator {
   virtual bool HasOverWrittenSingleDel() const { return false; }
 
   // Returns n where the current entry is the n-th update to the current key.
+  // The update count starts from 1.
   // Only valid if WBWI is created with overwrite_key = true.
   virtual uint32_t GetUpdateCount() const { return 0; }
 };

utilities/write_batch_with_index/write_batch_with_index_internal.h

@@ -165,7 +165,8 @@ struct WriteBatchIndexEntry {
   uint32_t column_family;  // column family of the entry.
   // The following three fields are only maintained when the WBWI is created
   // with overwrite_key = true.
-  uint32_t update_count;   // number of updates for this key up to this entry.
+  uint32_t update_count;   // The number of updates (1-based) for this key up to
+                           // this entry.
   bool has_single_del;     // whether single del was issued for this key
   bool has_overwritten_single_del;  // whether a single del for this key was
                                     // overwritten by another key