update comment of GetNextFile
update comment of GetNextFile

Summary: Closes https://github.com/facebook/rocksdb/pull/2377

Differential Revision: D5141274

Pulled By: lightmark

fbshipit-source-id: c237a285b73ad93488c080ea80c71a29a17f1be0

bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb