Fix the description of unordered_write in db_bench (#6476)

Summary:
As reported in https://github.com/facebook/rocksdb/issues/6467, the
description of the `unordered_write` switch of `db_bench` was incorrect.
(Note: the new description is based on
https://rocksdb.org/blog/2019/08/15/unordered-write.html).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6476

Test Plan: `db_bench --help`

Differential Revision: D20200653

Pulled By: ltamasi

fbshipit-source-id: 4c3683fcfa6a069164167af5aaff9974a810c16a

tools/db_bench_tool.cc

@@ -991,8 +991,10 @@ DEFINE_uint64(delayed_write_rate, 8388608u,
 DEFINE_bool(enable_pipelined_write, true,
             "Allow WAL and memtable writes to be pipelined");
 
-DEFINE_bool(unordered_write, false,
-            "Allow WAL and memtable writes to be pipelined");
+DEFINE_bool(
+    unordered_write, false,
+    "Enable the unordered write feature, which provides higher throughput but "
+    "relaxes the guarantees around atomic reads and immutable snapshots");
 
 DEFINE_bool(allow_concurrent_memtable_write, true,
             "Allow multi-writers to update mem tables in parallel.");