Improve Put()'s comment to indicate that the key is overwritten if existing

Summary: As title

Test Plan: Not needed for comment only.

Reviewers: yhchiang, ljin, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: xjin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19887

include/rocksdb/db.h

@@ -160,6 +160,7 @@ class DB {
   virtual Status DropColumnFamily(ColumnFamilyHandle* column_family);
 
   // Set the database entry for "key" to "value".
+  // If "key" already exists, it will be overwritten.
   // Returns OK on success, and a non-OK status on error.
   // Note: consider setting options.sync = true.
   virtual Status Put(const WriteOptions& options,