Update comments in DB::Close()

Summary: Closes https://github.com/facebook/rocksdb/pull/3543

Differential Revision: D7093251

Pulled By: siying

fbshipit-source-id: 4066b82c95ecb65866c5842d68ab13ab9f85d567

include/rocksdb/db.h

@@ -174,7 +174,8 @@ class DB {
   // Close the DB by releasing resources, closing files etc. This should be
   // called before calling the desctructor so that the caller can get back a
   // status in case there are any errors. This will not fsync the WAL files.
-  // If syncing is required, the caller must first call SyncWAL.
+  // If syncing is required, the caller must first call SyncWAL(), or Write()
+  // using an empty write batch with WriteOptions.sync=true.
   // Regardless of the return status, the DB must be freed. If the return
   // status is NotSupported(), then the DB implementation does cleanup in the
   // destructor