update max_write_buffer_number_to_maintain docblock

Summary: fix comment

Test Plan: n/a

Reviewers: sdong, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46455

include/rocksdb/options.h

@@ -276,10 +276,9 @@ struct ColumnFamilyOptions {
   // If this value is set to -1, 'max_write_buffer_number' will be used.
   //
   // Default:
-  // If using an OptimisticTransactionDB, the default value will be set to the
+  // If using a TransactionDB/OptimisticTransactionDB, the default value will
-  // value
+  // be set to the value of 'max_write_buffer_number' if it is not explicitly
-  // of 'max_write_buffer_number' if it is not explicitly set by the user.
+  // set by the user.  Otherwise, the default is 0.
-  // Otherwise, the default is 0.
   int max_write_buffer_number_to_maintain;
 
   // Compress blocks using the specified compression algorithm.  This